5 Reasons DataMapper Will Deprecate ActiveRecord is a well pointed out list of reasons why DataMapper will in the author's opinion overtake ActiveRecord. And there is nothing left to do but to agree.
I started a project a while ago, 1topf.net, which unfortunately died because of a massive lack of time here, but I started building it using Merb and DataMapper. Although the Merb version at that time was something like 0.9 and had some bugs and flaws, in combination with DataMapper it was a convincing team.
In my opinion, the Wedding of Rails and Merb will be a very exciting step in the Web-Framework universe. Let's see what happens, but the freedom to choose an alternative ORM is quite promising.