Found it. Total newb move on my part: I was trying to upload as if it were directly connected, not File->Upload Using Programmer.
I found the hint in a reply post by Riva in this thread: http://arduino.cc/forum/index.php/topic,131456.0.html
As I thought - it was a silly mistake. I'd been burning AT Tiny 85's a few weeks back with no issues, but this one step of the process I forgot!