Macs seem to be a "weapon of choice" for most Rails developers. Maybe because David Heinemeier Hansson, the creator of Rails, created and promoted Rails using a Mac, or possibly because of TextMate, the most popular text editor in the Rails community.
Another major reason might be that OS X is built on top of Unix, providing developers with a safe, stable, and a familiar environment.
Despite all this, the version of Ruby that shipped with OS X (Tiger), does not work well with Rails, and you will need to install an updated version of Ruby to get started with your Rails development. This is at least until Apple releases Leopard, their new OS, at the end of this month. Leopard will come with Rails pre-installed.
Hivelogic, a popular Rails blog, created an article, detailing the process of setting up your Mac to be an ultimate Rails development environment.