Does the latest stable Radiant version 0.9.1 (as of 2011.03.26) support the latest stable ruby / rails version (1.9.2p180 / rails 3.0.5)?
Is it safe to run Radiant 0.9.1 with 1.9.2p180 / rails 3.0.5?
Radiant doesn't support Rails 3 yet. Radiant are using vendor rails, which means the rails is fixed in the Radiant source code. However, there is a branch on Github for Radiant that supports Rails 3, but it is a work in progress.
As for Ruby 1.9.2, Radiant can't use it. When you create a Radiant project, in script/server
require File.dirname(__FILE__) + '/../config/boot'
This won't work, when your current path is in the root of the project folder, you can either cd to the script directory, then run
ruby server
Then it should work, else you have to change this script/server a bit. This is because . is no longer included in the $LOAD_PATH, so it will tell you that it can't find /../config/boot file (also see this Rails issue).
However, there are some other parts of the Radiant source core that are not really compatible with Ruby 1.9.2. Radiant recommend using ruby 1.8.6 and 1.8.7.