Who will Jennifer do?

It's a question that might have big implications for the late-night TV landscape. With the debut of NBC's The Jay Leno Show a month away, the late-night wars are coming to a fresh boil. Jennifer Aniston is one of the first A-list stars to have a movie opening after the September 14 bow of Leno, and her choice of couch to promote the romantic comedy Love Happens might foretell a shift in the ultra-competitive world of talk-show booking.

Or at least Leno producers hope so. During the past two weeks, they quietly have hosted teams from the town's top public relations firms for a tour of the new studio and a presentation touting promotional opportunities for their clients. The 10 p.m. show technically won't compete with traditional late-night offerings and will feature a slightly different format, but it is being treated as such a show for the purpose of booking top guests.

The message was made clear: 'We'd like you to do our show first,' says a top PR rep who attended a Leno pitch session but did not want to be named because she books clients on all the shows.

PLAYING-FIELD CHANGES

For years, the late-night booking hierarchy was relatively settled. NBC's The Tonight Show With Jay Leno and CBS' Late Show With David Letterman battled for A-list guests for their 11:30 shows. ABC's Jimmy Kimmel Live! had success with talent catering to a younger, male-skewing audience, and 12:30 hosts Conan O'Brien and Craig Ferguson were more flexible with invitations.

Now, with O'Brien inheriting Tonight and Jimmy Fallon installed at Late Night, the addition of Leno has opened a new front in the booking wars.

I've never seen it like this, says a late-night insider of the jockeying to secure guests. Now people are being much more strategic, and they are planning it out far more in advance.

NBC is in a particularly interesting position, competing not only with CBS and ABC -- and, to a lesser extent, with cable outlets like Comedy Central's The Daily Show With Jon Stewart -- but also between its own O'Brien and Leno shows.

Although the personal relationship between the current and former Tonight hosts is said to be friendly, Leno acknowledged the competition last week at the Television Critics Association's summer press tour.

Will we fight like cats and dogs to get the guest? Yes, Leno said. But that doesn't mean you don't like each other. It's a game.

A game with high stakes: Viewers often tune in to talk shows based on the tease announcing guests. One great guest, Hugh Grant in 1995, turned around Leno's fortunes at Tonight and propelled him to the late-night ratings lead.

'LENO' THE WILD CARD

With O'Brien and Leno attempting to establish themselves in new roles, scoring top talent is considered crucial. That's why the producing teams on both Leno and Tonight are acting aggressively.

NBC declined interview requests, but insiders downplay a potential booking rivalry, pointing to the longtime close relationship between Tonight executive producer Jeff Ross and Leno executive producer Debbie Vickers. The two coordinated guest bookings for years when their hosts were on Late Night and Tonight, respectively.

Although Leno is considered an untested wild card, several publicists say they walked away from the presentation excited about the show, especially the chance for clients to reach a 10 p.m. audience with a project pitch.

Even if Leno disappoints in the ratings, he will still probably double the audience of other hosts, one PR rep says. It's the primetime demo, the late-night demo and even the morning demo all rolled into one. That's pretty attractive.

Still, stars decide which shows to do based on many factors, including their relationship with the host, the audience demographic and which venue fits their schedule. The Los Angeles-based Leno is likely to affect Tonight, Kimmel and Ferguson's Late Late Show more than New York-based Letterman and Fallon. But celebrities are still going to go where they feel most comfortable, a network insider says. It's not just about ratings.

Rival network sources believe that Leno might have trouble booking stars of non-NBC shows because he competes with the primetime offerings on those networks, but talent reps suggest the format of Leno might help him land guests, with longer interview segments and the chance for actors to be incorporated into comedy bits.

Those include a 10 Questions-type segment and the advertiser-friendly Green Car Challenge, where celebrities will race alternative-fuel vehicles around a track NBC has built for the show.

Nobody knows how it's going to shake out, another publicist says of Leno. They are saying they won't necessarily forbid someone from doing Conan and then Jay, but it would probably be in one of the smaller (non-interview) positions.

The week of September 14, which provides an unusually high number of actors promoting projects, will be instructive. In addition to Aniston pitching Happens, which is being released by NBC sister Universal Pictures, Matt Damon has The Informant! opening that Friday and has agreed to do a Kimmel comedy bit. Megan Fox will be promoting Jennifer's Body, and the voice cast of the animated Cloudy with a Chance of Meatballs includes Andy Samberg and Bill Hader of NBC's Saturday Night Live.

Charlize Theron will appear on Tonight on September 14. Leno has said his first show will include a performance by Jay-Z with Rihanna and Kanye West, but other bookings are being kept under wraps.