I've just updated the text on the Gentoo Wiki page on Ruby 1.9 to indicate that we now support eselecting ruby19 as the default ruby interpreter. This has not been tested extensively, so there may still be some problems with it. Please open bugs if you run into problems.
Most packages are now ready for ruby 1.9. If your favorite packages are not ready yet, please file a bug as well. We expect to make ruby 1.9 the default ruby interpreter in a few months time at the most. Your bug reports can help speed that up.
On a related note, we will be masking Ruby Enterprise Edition (ree18) shortly. With Ruby 1.9 now stable and well-supported we no longer see the need to also provide Ruby Enterprise Edition. This is also upstream's advice. On top of this the last few releases of ree18 never worked properly on Gentoo due to threading issues, and these are currenty already hard-masked.
Since we realize people may depend on ree18 and migration to ruby19 may not be straightforward, we intend to move slowly here. Expect a package mask within a month or so, and instead of the customary month we probably won't remove ree18 until after three months or so. That should give everyone plenty of time to migrate.