I just published an article explaining how Ruby Object Mapper works internally. If you're interested in writing ROM adapters or just curious to understand "the magic" behind ROM go and check it out. It's right here.
I've put together some of my thoughts about current state of Ruby ORMs, mentioning ActiveRecord, DataMapper and Sequel. Take a look if you're interested in ORM development efforts: The State of Ruby ORM
If you're migrating Merb applications to Rails3 or you're about to start doing that then make sure you check out a helper library we've written recently called merb-to-rails3 which will ease the (small) pain of the migration.