While I'm confident enough poking around CentOS (and other RedHat derived distributions), Debian and it's ilk are definitely my preferred option.
I'd not spend too much time on Sarge (3.1) as some suggested above as I think that its security support window expires early next year, but Etch (4.0) will be around longer.
While I prefer the Debian for server use (out of habit if nothing else), hence it got my vote, you might be better off going for Ubuntu's LTS variants. This is currently Dapper Drake only, though depending on how long you expect any project to support Ubuntu to take (or even take to start) there may be a second LTS version active by the time you release - I think Heron, coincidentally due to be released about the same time as Debian/Sarge is due to be fully retired, will be the next release with the LTS promise.
Dapper should be supported for official security updates until some time in 2011. Etch probably won't be actively supported that long, and the next release (Lenny) is most likely quite a moving target for at least another few months.
Not that my opinion counts for a lot, as ATM I'm pretty much leaving the hosting racket for the time being and the single VPS license that is the only one I have left running doesn't amount to a significant sum...