ActiveModel is out now with Rails 3.... so when should we use it?

[Travis Reeder]

Using ActiveModel will enable a lot of stuff in SimpleRecord that we won't have to build ourselves so I think we should definitely try to use it. That said, it requires active_support 3+ which in turn requires rails 3+ so I'm thinking the best way it so only enable it if it's running in a Rails 3+ environment or if not in rails, the user has the active_model gem installed.

Anyone know off hand how we could check for those conditions before requiring gems and what not?