It's easy to see why Richard Rodgers and Lorenz Hart's song "The Lady Is a Tramp" became a standard. Its long, flowing lyric is easy to ad-lib around, and can be sung by a woman (in the first or third person), by a man, or as a duet with very few alterations. It's packed with specific references to the New York City of the Thirties, but most of them, astonishingly, are still comprehensible in 2011. Its melody is very forgiving for limited voices, and gives suppler voices opportunities to show off.
One surprising aspect of Tony Bennett and Lady Gaga's duet version of the song is that 85-year-old Bennett is the one playing straight man, and Gaga's the daredevil, showing off, interjecting jokes, switching between speaking and singing voices, and generally keeping Bennett on his toes. (Compare it with Bennett's duet with the late Amy Winehouse on "Body and Soul": Winehouse limits herself by sticking to a Billie Holiday impression, while Bennett finds poignant hints of a May-December romance in its lyrics.) Gaga and Bennett only get through the first two verses of "The Lady Is a Tramp," and tweak a lot of the lyrics, although their only major alteration is changing a bit about journalist Walter Winchell to a tribute to the song's composers: "I follow Rodgers and Hart—" "She sings every line!"
Rodgers and Hart introduced "The Lady Is a Tramp" in 1937, in their musical Babes in Arms (the same show that introduced "My Funny Valentine"). Tommy Dorsey and His Clambake Seven recorded what might be the earliest surviving version of it: it's mostly an instrumental showcase, in which Edythe Wright shows up after a minute or so to sing a couple of verses. Sophie Tucker sang a more complete version the same year. Then it disappeared from view until Lena Horne revived it in 1948, in Words and Music, a film musical about Rodgers and Hart's partnership; for perspective, that would be the equivalent of, say, "Try Again" turning up this year in an Aaliyah biopic.
What really got "The Lady Is a Tramp" into the pop canon seems to have been Ella Fitzgerald's performance on her 1956 album Ella Fitzgerald Sings the Rodgers and Hart Song Book. It was a quick follow-up to her very successful Cole Porter Song Book album from earlier that year—she ultimately recorded eight "songbook" albums devoted to American pop composers—and, as with a lot of those performances, that version of "The Lady" was more about showing off the unadorned glory of the song than imposing her own personality on it. (Later on, she'd have more fun with her interpretations.)
But it was Frank Sinatra who gave the song an even bigger boost. Apparently, he insisted in performing it in the 1957 film of another Rodgers-and-Hart musical, Pal Joey (in which he dropped the song's introduction, with its explanatory lines about "Mulligan stew"—hobo food—and "hobohemia"). It stayed in his repertoire for the rest of his life; he and Fitzgerald sang it together on a 1967 TV special. After Sinatra started giving "The Lady Is a Tramp" play, it got more currency among other performers: one particularly odd version is Jaye P. Morgan's 1958 rendition, backed up by the great vibraphonist Lionel Hampton. Shirley Bassey kept it in her repertoire for decades, too (here she is singing it in 1966). It's even turned up in a few rock singers' repertoires: see, for instance, Alice Cooper's awkwardly straight 1973 version or Nina Hagen's bug-eyed 2004 take.
The intersection of rock and Sinatra is also responsible for one of the oddest versions of the song. When the Beatles launched their label Apple Records in 1968, three of its first four public releases bore serial numbers APPLE 2, 3 and 4—but not 1. (The fourth was "Hey Jude," which was numbered as an EMI/Capitol release). That's because APPLE 1 was Sinatra's performance of a rewritten version of "The Lady Is a Tramp," retitled "Maureen Is a Champ," and privately pressed (there were only a handful of copies made, and it's never been officially issued). Ringo Starr's wife Maureen was a Sinatra fan, and as a birthday present to her, Apple commissioned songwriter Sammy Cahn to write a new set of lyrics ("She married Ringo, but she could've had Paul...") and Sinatra to sing them.
The other surprise about Bennett and Gaga's version of "The Lady Is a Tramp," though, is that it's entered Bennett's repertoire this late. He's sung it before, of course--no vocalist of his kind could get away without singing it occasionally. Bennett recorded a version on his 1992 Frank Sinatra tribute album Perfectly Frank, and the two of them sometimes performed it as a duet, as with this 1988 performance. A year after Perfectly Frank, Bennett also sang it with Natalie Cole at the Grammy Awards—they do their best to smile through it. Still, it doesn't seem to be a song that means a lot to either of them. For Gaga, it's a different story. "The Lady Is a Tramp" is a different kind of singing than she's gotten to do on record before, and she's clearly thrilled and a little nervous to have a chance to show off her chops. Having an old master singer beside her to offer his seal of approval spurs her on even more.