The Labor Day box office may have been sluggish — with none of the new releases doing big business — but the summer ended on a high note in all but matching last year’s record-setting B.O. haul.

Through the Monday holiday, summer box office was estimated at roughly $4.12 billion vs. $4.16 billion for summer 2007. Year to date, the domestic box office is down about 1% vs. last year. Final tallies will be released today. Attendance is down by about 3%-4% for the summer and the year to date.

The other big headline of Labor Day — and the entire summer, for that matter — was Warner Bros.’ “The Dark Knight,” which jumped the $500 million mark Sunday to become only the second film in history to accomplish that feat after “Titanic.”

Studio projects that the Batman sequel will reach $530 million domestically; “Titanic’s” status as the top-grossing pic of all time remains safe with a domestic gross of $600.8 million.

The Labor Day frame saw five new releases, but DreamWorks and Paramount’s holdover comedy “Tropic Thunder” won the frame for the third weekend in a row, grossing an estimated $14.3 million from 3,473 theaters. Cume is $86.6 million.

Coming in No. 2 for the weekend was 20th Century Fox’s Vin Diesel sci-fi actioner “Babylon A.D.,” which grossed an estimated $12 million from 3,390 theaters. Pic came in on the lower end of expectations.

Sesh was down by as much as 17% from last Labor Day, when Dimension’s “Halloween” remake opened to a boffo $26 million.

Arguably the one bright spot among the new openers was Overture’s Don Cheadle-Guy Pearce political thriller “Traitor,” which played to older auds gearing up for more serious fall fare. The first inhouse production from Overture grossed an estimated $10 million from 2,054 runs for a six-day cume of $11.5 million (“Traitor” opened Wednesday). Per-location average of $4,869 was more than that of any other film in wide release.

Woody Allen’s “Vicky Cristina Barcelona” also fared well in appealing to an adult crowd. “Barcelona,” from the Weinstein Co. and distributed by MGM, grossed an estimated $3.5 million from 692 runs for a cume of $13.3 million in its third weekend. Film placed No. 10.

Three new Labor Day comedies couldn’t generate many yuks. Lionsgate’s “Disaster Movie” spoof, from “Scary Movie” team Jason Friedberg and Aaron Seltzer, opened at only No. 7. Pic, about the making of a disaster film, grossed an estimated $6.9 million from 2,604.

There were even fewer laughs for Element Film’s “College,” distribbed by MGM, and Focus Features’ “Hamlet 2.” Both opened outside the weekend’s top 10.

“College” grossed $2.1 million from 2,123 for a per-location average of $992.

“Hamlet 2,” which Focus opened wide after one weekend in limited release, grossed an estimated $2.1 million from 1,591 for a per-location average of $1,324 and a cume of $3.1 million. Sobering results for the film, acquired by Focus at Sundance, come just as the indie biz gears up for the Toronto Film Festival.

If the appetite for specialty titles was virtually nonexistent at the summer box office, commercial fare thrived, beginning with Paramount and Marvel Entertainment’s Robert Downey Jr. starrer “Iron Man,” which kicked off the summer sesh and cumed $317.6 million. Memorial Day tentpole “Indiana Jones and the Kingdom of the Crystal Skull,” also a Paramount release, grossed $315.3 million.

But it was Warners that ended the summer with a narrow lead in market share over Par, thanks largely to the runaway success of “Dark Knight,” from Christopher Nolan. No one expected the Batman sequel to reach the heights it did given its darker themes and the fact that Nolan’s “Batman Begins” topped out at $205.3 million domestically. But the film has appealed to all moviegoing demos and generated a tremendous amount of repeat biz.

“Everything clicked,” Warners prexy of domestic distribution Dan Fellman said.

Large-format exhib Imax has seen record-breaking ticket sales of $42.6 million for “Dark Knight.” Nolan shot several sequences with Imax cameras.

“Dark Knight,” which opened July 18, accomplished another feat in proving that the summer box office can remain strong across the board, not just in May and June. Last summer, theater owners complained that Hollywood frontloaded the season, throwing the season out of balance.

The summer was notable as well for birthing several likely new film franchises — “Iron Man” for Par/Marvel, “Get Smart” for Warners and “Sex and the City” for Warners/New Line, among other possible titles.

There was an overpopulation of comedies this summer, with two –Paramount’s Mike Myers starrer “The Love Guru” and Fox’s Eddie Murphy starrer “Meet Dave” bombing. But the Judd Apatow factory enjoyed far better success with the Sony pics “Pineapple Express” ($76.4 million to date) and “Step Brothers” ($96.6 million).

“Tropic Thunder” has performed strongly as well, although the film cost at least $90 million to produce.

Female-skewing pics have prospered, even through Labor Day, which has traditionally been a time for male-oriented movies and horror titles. Sony holdover “The House Bunny” placed No. 4 for the frame, grossing an estimated $10.2 million from 2,714 for a 10-day cume of $29.7 million. Universal’s “Mamma Mia!” actually saw a 2% bump thanks to special Labor Day sing-alongs. Universal had a busy summer, coming in No. 3 in market share and turning out a bevy of midrange successes, including “Wanted” and “The Incredible Hulk.”

Fox had a rough summer, with “Meet Dave” cuming just $11.6 million. “Babylon A.D.” returned the studio to prominence on the box office chart for the first time since June.

“We’re thrilled. We’re glad to be the No. 1 new opener of the weekend,” said Fox senior VP of distribution Bert Livingston.

Overture was altogether upbeat about the performance of “Traitor.”

“We think we are well positioned to play into the fall, because we know the word of mouth is good. It is a thriller that audiences are responding to,” Overture exec VP of distribution Kyle Davies said.

Likewise, Weinstein Co. distribution topper Steve Bunnell said “Vicky Cristina Barcelona” should continue to play. Pic has already eclipsed the $10 million domestic gross for Allen’s last pic, “Scoop.” The helmer’s “Match Point” grossed roughly $23 million domestically, and TWC believes “Barcelona” should reach that number.

Among specialty Labor Day openers, Sony Pictures Classics’ “I Served the King of England” grossed an estimated $50,096 from eight runs for a per-location average of $8,488.