Matthew Groves has been working on a series of articles comparing some OR/M frameworks. So far, he's done some posts on Entity Framework and Fluent NHibernate, but I believe he's planning on hitting a couple more, too.
If you're a .Net developer, you know what a "lively" subject this always is. Data access consumes so much time and effort in most applications that it's worth educating yourself on the options so you can make informed decisions. I'd also recommend checking out a couple of these frameworks yourself, as merely reading about these frameworks isn't quite the same as using them on a project.
Here's a list of Matthew's posts so far: