TDD and MVC Crossroads. What ORM integrates nicely?

Question

I'm just starting out in MVC world, and I've used a standard MVC template that's built into VS 2010. I've got a couple of controllers and views hooked up, and now I need to get to my database.

I've got an existing SQL Server 2005 database that is quite large, already defined by a 3rd party company. I'm trying to bolt on a reporting/admin interface to it for our specific company needs.

So, I'd like, initially to just pull a list of things out of a few tables from this DB. So...using MVC 3, what are some ORM tools that integrate nicely?

I'm also trying to use a Test Drive Design approach. I'm not sure what to do for tests that would require insert/update/delete of data. Is that were "Mocks" come into play?