On Mac OS X 10.6 (Snow Leopard) I had not problem installing rvm or ruby 1.9.2 but it blew up on my when I tried to list the gems under 1.9.2. The solution was to install zlib under the rvm tree:
$ ruby -v
ruby 1.8.7 (2009-06-12 patchlevel 174) [i686-darwin10.2.0]
$ rvm package install zlib
$ rvm install 1.9.2 -C --with-zlib-dir=$HOME/.rvm/usr
$ rvm use 1.9.2
$ gem list
$ gem install rails --pre
I had a similar problem when installing ruby 1.8.7 under rvm. This time it was the readline library that was the problem. Again, if you poke around in the docs you can find a fix:
$ rvm package install readline
$ rvm install 1.8.7 -C --with-readline-dir=$HOME/.rvm/usr
Although rvm makes it easy to switch back to your system ruby (with 'rvm system') I would strongly suggest that you do not use this. Instead, install the same release as your system ruby under rvm (1.8.7 in my case) and make that the default ('rvm --default 1.8.7').
You will have to reinstall gems under that ruby installation, which is a pain, but once you have all your rubies under rvm then you should have no issues with confusion over where specific libraries are located, which has been an issue for me more than once.
rvm also has a notion of gemsets, which I have not explored as yet, which allow you to isolate specific gem versions for specific applications. This could be extremely useful in migrating Rails applications to newer versions of Rails.
One area where I have not yet been successful is getting rvm and passenger to work. The rvm docs explain how to do this - but no luck for me as yet.