Pity CBS. David Letterman’s exit from the latenight stage should be fodder for ticker-tape parades, a classic promotional goldmine. Yet until now, the network has largely been handcuffed by the “Late Show” host’s taciturn nature, thus far keeping the cheers to something short of a roar. That appeared to turn a corner Monday, though, with a pretty potent one-two punch: A classy 90-minute primetime Letterman retrospective; and a latenight appearance by President Obama. Letterman doesn’t sign off until May 20, but that combination served notice that the end, professionally speaking, is near.

Admittedly, 90 minutes hardly seems like enough time to do Letterman’s 33 years as a latenight host justice, and perhaps understandably, clips from his NBC program, which occupied the first decade, were used sparingly. Moreover, despite Ray Romano’s warm stint hosting the special – “I’m only here because of David Letterman,” he said, referring to Letterman’s role as producer of “Everybody Loves Raymond” – there was a rather significant donut hole given the absence of Letterman himself.

For all the obligatory highlights, from “Stupid Pet Tricks” to musical guests, from funny animals to unpredictable celebrity moments, the special actually peaked in the final third. That section really captured the scope of Letterman’s career, including those guests, like Tom Hanks, Julia Roberts, Bill Murray and Howard Stern, who have aged but remained in the spotlight along with him. And it also reflected Letterman’s unique depth, from his return after the Sept. 11 terror attacks to his tribute to the medical personnel who treated him after his bypass surgery.

Finally, the producers (including longtime CBS latenight exec Vinnie Favale and Susan Zirinsky) illustrated the profound influence Johnny Carson had on Letterman, with Romano articulating how Letterman came to represent much the same thing for his generation of comics. The sequence reinforced the notion that while Letterman might not have inherited “The Tonight Show,” there was both a lingering bond and something of a karmic baton pass between them.

Letterman exhibited a sense of looseness during the monologue and his interview with Obama that suggests these next few weeks will be something to see. If the host has at times appeared disengaged during the last few years, particularly with guests, he’s clearly zeroed in on the finish line.

The Obama interview saw Letterman engage the president in a fairly detailed discussion about racial strife, Baltimore and the roots of poverty, then segue into a discussion of free trade. The opening and closing banter was light, but the discussion in between was anything but.

“Unlike latenight talkshow hosts, I am term limited,” Obama quipped, closing by saying, “We’ve grown up with you. … You’re part of all of us.”

The primetime special, rather cleverly, was bookended by R.E.M.’s “It’s the End of the World as We Know It” and David Bowie’s “Changes.” That’s a bit of hyperbole, since the world will go on. But after May 20, in the same way Carson left a very long shadow, latenight won’t quite be the same.