"This is the only time we get to do the Smurfs' theme song," declares Ahmir "?uestlove" Thompson, drummer and bandleader of the Roots. "So let's do it!"

And with that, the most respected group in hip-hop flashes back to some distant Saturday morning, reciting those cloying la-la-la-la-la-las, as if lost in a Frosted-Flake-induced trance. Strangely, this is the sound of a group at the top of its game.

Since signing on as the house band for "Late Night With Jimmy Fallon" a year ago, the Roots have morphed into an exciting new musicmaking machine — not to mention the best reason to keep your television glowing past 11:30 p.m.

With the record industry in shambles and the touring market growing more overcrowded each minute, the move to television seems particularly shrewd. But the fact that the Roots have been able to turn a Monday through Friday gig into their own creative playground seems just plain lucky.

On a recent Friday at 30 Rockefeller Plaza, the Roots are huddled into a tiny rehearsal space, finalizing the Smurfish walk-on music for the evening's big guest, actor Jude Law. (Law-law-law ... get it?)

Just four more episodes until holiday break — a break the Roots will spend playing concerts in California, Nevada, Oregon and Washington.

This marks the band's first appearances as television personalities. It's a metamorphosis that Thompson, 38, hasn't fully accepted.

"Oh, our (live) show is tighter than a mother," he says. "But there's the occasional identity crisis thing. You walk down the block and an older person might say, 'Oh, Jimmy Fallon's band!' still not knowing the history."

The band first took shape in the late '80s when Thompson and rapper Tariq "Black Thought" Trotter crossed paths at Philadelphia High School for Creative and Performing Arts. By 1993, the group had made a name for itself, bringing live instrumentation to a genre dominated by samplers and drum machines. Eight albums later, the Roots are recognized as one of the most dynamic outfits in pop music, backing Jay-Z one day and playing with the Flaming Lips the next.

But news of the band's latest undertaking left fans and critics highly skeptical. Did the Roots really plan to waste their evenings cranking out commercial bumper music in ill-fitting tuxedos? A year later, Thompson is finally coming out of his defensive crouch.

"They can kiss my (behind)," he says. "This is the best gig ever!"

Much of that has to do with the seemingly limitless creativity the Roots bring to the table. They've reinvented the art of walk-on music, personalizing songs for each and every guest with a wink, a jab or a nod of respect. The band's encyclopedic knowledge of pop comes in handy here, as does its impressive sense of humor. Among its best audio-japes: Joan Rivers was once treated to Lady Gaga's "Poker Face," while reality-television detritus Heidi and Spencer Pratt were serenaded with Beck's "Loser."

So while Fallon still finds himself in a ratings duel with Jimmy Kimmel and Craig Ferguson, the Roots have made "Late Night" the most buzzed-about show in music circles. "We've had established artists who've wanted to ditch their own bands and play with the Roots instead," says Jonathan Cohen, Fallon's music booker. "They set the show apart from our competitors."

Despite the host's support, the band had to hustle to win the trust of the suits at NBC. At Thompson's behest, the show's writers developed two sketches that showcased the band's talent: "Slow Jam the News," in which Fallon recounts the day's headlines over a sultry R&B groove, and "Freestylin' With the Roots," in which Fallon nabs a random audience member, asks him a few questions and then asks the Roots to compose a quick tune about him on the spot.

The success of these skits has helped the band cement a five-year contract that gives Thompson and his troupe the freedom to redefine what a late-night house band can be.