Road bike carbon fiber frame life span
Absent some kind of abuse or extraordinary stress (like falling on top of the bike while it's lying on the ground - stressing the tubes from the side, riding with a friend sitting on the top tube...) a carbon fiber frame should outlast the rider.

Lennard Zinn covered durability of Carbon Forks for Velo News a while back.  Here are some quotes from that article in case the link breaks:

From Kestrel


  For carbon forks in
  general, there should not be any
  limited life span, as carbon
  composites themselves are not subject
  to fatigue failures as metals are. So
  the fatigue life of a properly made
  carbon composite is 'infinite'.


From Look


  There is no limitation because carbon
  has a natural flexibility. It can be
  used a hundred years while maintaining
  the same stiffness.


From Easton


  The good news is the fatigue life of
  carbon fiber is immensely more than
  that of metals. While the writer
  expresses concern about his carbon
  fork lasting as long as a metal
  component, there is nothing to worry
  about in terms of fatigue life on a
  composite fork.


Having ridden since the early 80's I've been through this twice, once with aluminum frames and now with carbon fiber. As high end competition bikes were made of the materials there were questions of durability.  Over time, as the frames became widely available, those concerns were shown to be largely baseless.

I believe most frame builders warranty carbon frames for life now. I personally know of at least two riders with carbon fiber bikes that have been ridden in excess of 100,000 miles.

That said, you should periodically inspect the frame for cracks and wear, especially at stress points like around the bottom bracket (just like you would with any frame).