Well, looks like NBC is trying out a show this fall (Coupling, based on a show in the UK) that looks very similar to Friends, 6 young attractive singles trying to find their way, except it's a lot raunchier, with much more sexual innuendo, than Friends. And the 6 cast members are 30-somethings rather than 20-somethings (as it was when Friends started out). Perhaps the NBC execs are trying to groom Friends' eventual successor, with the perennial hit show going off the air after next year.
However, when Friends goes off the air, it's unlikely Coupling will replace Friends in the 8pm Thursday timeslot unless they tone it down a heck of a lot. According to articles I've read, it's so sexually explicit that it looks more like a show you'd see on HBO rather than on network TV. This fall they'll be showing it at 9:30 pm on Thursday, where most of the young kids will be in bed. It's going to be a tall order to compete against the 2nd half of "CSI," but then again, "Good Morning Miami" survived just fine.
But I can't think of any other current NBC comedy (Scrubs, Will & Grace, Good Morning Miami) that would be able to replace that would appeal to so many demographics and draw such huge ratings as Friends does, so maybe a scaled-back Coupling will replace Friends at 8pm, who knows.
-Frank
P.S. Frasier is going of the air after next year, too, so it obviously won't be able to replace Friends.