Hi,
I'm glad that you like Catnap. I have been using it for some time in my two MonoTouch apps, and it meets my needs quite well.
Regarding IEntity, I understand your point. When I developed Catnap I knew that I did not want an ActiveRecord type of pattern where entities must derive from a heavy concrete base class. From there I was faced with a tradeoff between a very small dependency (a very light interface) and good bit more complex implementation. So I chose the former. I am very much open to breaking this dependency if it could be done with minimal impact on backward compatibility. That said, it's probably not something that I will personally take on in the near future. If you are interested in pursuing that I would welcome the contribution.
By the way, I recently used Catnap for a Windows native client with an embedded SQLite database, and it works quite nicely for that too!
Tim Scott