Of course it's Wallace.

The last episode of Veronica Mars solved lots of mysteries posited during the season. But it ends with a cliffhanger, as Veronica opens the door and says, "I knew it would be you." Is it Old Boyfriend Duncan? New Boyfriend Logan? Goofy Hacker Chick? Nah, it has to be Wallace. Makes a nice bookend when you remember the very first episode, as she saved him from duct tape crucifixion. Besides, it's like 3 a.m., and who else would know but Wallace (since Veronica called his Mom to come stay with her Dad). Lots of other loose ends from that episode though, so I'm anxiously looking forward to next season.