It appears that the next version of Linux Mandrake will have Ruby in it; they already have ruby-1.4.5 (binaries, docs, and extensions) ready to go.  There is also mod_ruby (for Apache).

I didn't see it in the Red Hat 7.0 upcoming release (Pinstripe).

I just generated my own Red Hat 6.2 derived CDROM, so I found this out :-)

Now I'm writing tools to generate Red Hat derived CDs using Ruby (as well as RPM tools).