One of the things I get asked quite often is "How does LINQ to SQL affect performance compared to writing my own Data Access Layer?" LINQ to SQL perf is a work in progress of course since it is still in beta. However, if you are curious, check out Rico Mariani's first three of a series of post on LINQ to SQL performance:
DLinq (Linq to SQL) Performance (Part 1)
DLinq (Linq to SQL) Performance (Part 2)
DLinq (Linq to SQL) Performance (Part 3)
After you read the first three, you should head over to Matt Warren's blog to see what he has to say. Of course the real answer you are probably looking for hasn't been revealed yet, but stay tuned to Rico's blog.