How long do lenses last?

I have a 70-200 2.8 L USM that I bought new in 2006.  I take good care of it (always covered and in the bag when not in use).  I used it with my 30D and now my new 7D.  Everything seems to be okay-there are some shots that are soft, but my guess is that's due more to user error than anything.

Does there come a point from a time perspective as to when you look to replace a lens?  If this lens is in good shape would I notice much of a difference with a newer version of it?  I use it mainly for sports.

Thanks.