When we announced Rails support for the 10gen cloud computing platform, we said that ActiveRecord support was not yet included. This led naturally to the question, "what the heck good is that?"
Actually, you can run Rails without ActiveRecord just fine. Rails has been designed that way. For an example app that uses the Mongo database, see my 10gen-rorob Rails app. You might want to use the branch that uses Rails 2.2.2 instead, because the master branch uses Rails 1.2.6 and the 2.2.2 branch also has a few more features.
On the other hand, internally we have a subset of ActiveRecord working already and are working on supporting as much of it as makes sense with a non-relational database. I already have the Rails app from "Agile Web Development With Rails" running unchanged.
On the gripping hand, you can run the 10gen cloud yourself, in which case you can run it with MySQL or any other relational database. In that case, you can use ActiveRecord unchanged.