Installing the latest upgrade from Apple of Snow Leopard version 10.6.3 will update the installed version of Ruby.
The version of ruby installed is:
Unfortunately, there is a known problem with patchlevel 173.
Under the previous version of ruby that was shipped with Snow Leopard, you would have received the expected answer of 3.0009.
There is a patch that you could try in the short term, otherwise you're probably best advised to hold off installing 10.6.3. Or, if you've already installed it, you can wait until Apple or someone else releases an update.