No. It is not.
It has been in a state of crisis for some time now, though. And that state of crisis is partially due to the form being too predictable (type of gags, joke rhythms - many have been used since the beginning of radio comedy in the 1930's).
Hence the rise of the single-camera, more filmic approach to sitcoms. Which has resulted in some excellent series (Spaced, Scrubs...) and a far larger palette of comedic effects (soundtrack, editing, quoting famous films etc.).
But the traditional, studio-bound sitcom is apparently once again on top of the ratings in the US (see Ken Levine's blog for more on this). Shows like How I Met Your Mother, The Big Bang Theory etc. are doing very well indeed. And, more importantly, none of the current crop of sitcoms is really classic material.
So what is needed for a new major sitcom revolution? A major hit with cultural impact.
And how do you get such a hit? No one knows (or there would never have been a sitcom crisis in the first place).
Yet if we look at the really big US hits, there are some elements which they have in common:
1) There's nothing else like them on air when they start.
Cheers, All In The Family, Sanford and Son, Frasier, Seinfeld, Friends, Everybody Loves Raymond: all of these series have a very distinct tone. Which was copied in many cases once the series became a hit, but when it started, it offered a new approach to the genre. Cheers offered a very mature, edgy look at romantic relationships (as well as a quite spiky level of anti-intellectualism and misogyny at times); Seinfeld had a style and rhythm completely its own and built storylines out of 'nothing'; All In The Family broke TV taboos at an alarming rate.
2) They provide surrogate families for the audience to become a part of.
This is really self-explanatory. Just look at the list.
3) They have a unique voice.
It bears repeating: each of these series has a very clear creative direction, born from the passion of its creators and their specific talent. Too many failed or uninteresting sitcoms feel like carbon copies of each other. Talent must be given the freedom to create and to foster its uniqueness.
4) They have an edge.
Even though there's this surrogate family vibe going on, the comedy isn't all sweetness and light. Sam and Diane destroy each other; Frasier is an unhappy man despite all his good intentions and psychological know-how; the Friends all have their own self-destructive quirks and can be horrid to people not part of their inner circle; Archie Bunker is a tyrant to his long-suffering, loving wife Edith.
Granted, series like The Cosby Show and Diff'rent Strokes avoided any unpleasantness. But they haven't aged nearly as well as the other shows we're talking about.
5) A huge portion of the audience can relate to the basic set-up/conflict.
6) They capture the zeitgeist.
Not by being about a trendy topic (an internet startup company) but by reflecting current attitudes towards relationships between people and towards societal norms. Since these do not change quickly, this ensures that the series can run for a fair couple of years. Being ahead of the crowd can be dangerous, though: Ellen worked fine when it was about a thirtysomething woman looking for her place in life. When the character came out of the closet, that was a huge ratings success; however the season which followed and which showed Ellen adopting the gay lifestyle was too far ahead of the general audience. A similar development in a comedy series today would be accepted far more quickly.