I looked through the code quickly, and it seems to rely on ActiveRecord for the History model.
However it probably wouldn't be too difficult to make an ORM-independent version of this, if that's the only place ActiveRecord is used. An admin tool really only needs to use the models that you have created elsewhere in your project. So they could make the history part of this configurable and allow you to supply your own model to handle those functions. It would be like how most authentication modules let you write your own User model, as long as you put a 'magic include' in that model to pull in the authentication stuff.
It very simple to write a quick DataMapper adapter for RailsAdmin. The thing with the history model is going to be changed probably on Saturday's BugMash.
