Any decent and up to date Linux should be fine though - the important
thing being a decent package management system with up to date
packages for php, mysql, imagemagick, ghostscript etc. so you don't
have to compile anything from source.
For development I use Mac OS X, and this works so well it could be
used for production too but it wouldn't be as optimised as Ubuntu
because there's no way to disable all the GUI stuff in OS X (at least
I don't think there is). Still, with no-one logged in this should be
pretty minimal.
Dan