On the ending...
I actually had a feeling earlier in the season that Trinity would target Rita before he even met Dexter. She just fit the profile of the women he went after -- mid to late 30's with kids, etc. -- just didn't know if she was a bathtub death or a jumper. Once it seemed like the kill cycle was over I thought she was "safe" until Trinity found out Dexter's true identity.
On a sidenote, the conversation between Michael C. Hall and John Lithgow after the show/before the Californication finale was pretty interesting, too. I was a little surprised at the way Hall looked, though -- don't know if it's going to be a new look for Dexter, if he's working on another movie at the moment, or just wanted to shake things up while he's not in character, though.