I think Rails itself works, the problem is the servers. Mongrel doesn't work on 1.9, and the others have problems too. Also the DB libraries.

Plenty of other gems don't work either, basic ones like JSON (works only in pure Ruby mode). Hopefully the stable 1.9 release spurs maintainers on a bit, but I imagine it'll take a while.

Then there are the DB libraries that only work in 1.9 like NeverBlock.

in the rails 2.2 release notes, it did mention 1.9 compatibility.