As a Rubyist, I hack on a Linux desktop and a MacBook Pro. On my desktop, I'm running Gentoo unstable, Snow Leopard 10.6.4 on the MacBook Pro. It's been my experience, however, that Ubuntu tends to dominate the community of Rubyists using Linux.

That said, OSX seems to be the most popular choice among developers, followed by Ubuntu/Fedora/CentOS.