Rob Davis has been the Packers long snapper since 1997 and the 40 year old is expected back in 2008. I doubt he's ever played on offense, but I can't remember a bad snap on a punt and he's even pretty good at getting down field on kick coverage. The Packers' punting unit collapsed back in week 16 at Chicago, but it wasn't because of Davis.
Although he's avoided a lot of wear-and-tear by only playing on special teams since 1997, his longevity is still amazing. If it were that easy to avoid injury by just playing on special teams then there would be a lot more players in their 40s still playing on special teams.