Thanks for the reply. I think it's the key because that's the only variable - all I've changed is the key fob batteries. The car started before I changed them, won't start after I changed them. It seems far less likely to me that a random component would fail when the car's parked in the garage, at the exact moment I change the batteries in the fob.

You say "The batteries only support the lock / unlock function" but I understood the fob is also synced to the anti-theft system, which would imply that there's a chip in the fob that has to communicate with a similar one in the car. I'm probably wrong in this assumption, but it seems logical to me. Therefore, the fob batteries would also be necessary for that chip, would they not? Obviously, all my assumptions may be wrong, and I'll be the first to admit that, but I'm just trying to wade through this quagmire as best I can.