There's currently a poll on TLC discussing the question, "How will the relationship between a wizard and his wand become more important in Book 7 (as JKR said it would at Radio City Music Hall)?"
Looking at the possible answers to the question, reminded me of a theory I had shortly after reading Half-Blood Prince. At the time I was thinking, "Why would Voldemort want Olivander? Voldey already has a wand."
Although there does appear to be room for legitimate debate, I never thought that Olivander seemed the sort to become a Death Eater. From the very first I assumed he had been taken either by force or threat. And, while it's completely possible that he was taken just to weaken the good guys, I think there's something much more sinister afoot. I think he was kidnapped to make Voldemort a new wand.
While I'm sure Voldemort very attached to his old wand, it has one horrible defect if you're Voldemort--i.e. the core feather from one Fawkes the Phoenix who happened to give one other feather that now resides in the wand of one Harry Potter. Because if you think about it, it's going to be really difficult for Voldemort to kill Harry as long as both have Fawkes feather wands. Harry could answer Voldey's AK with any super simple spell--it doesn't even really seem to matter what. The Priori Incantatem effect will occur, and--wa lah--graveyard deja vu. Unless Voldemort or Harry catches the other completely by surprise, it will be almost impossible for them to kill each other unless one gets a new wand.
Alternatively, maybe Voldey just kidnapped Olivander for information. After all, he's the best, and he probably didn't get to be the best without knowing a few things about how wands work. Maybe Voldemort is tapping Olivander's knowledge to figure out a way around PI.