Ha - ok this is a first. Since my OP, the problem is the exact opposite. I put something in the trash now and the can stays empty. It's not a custom icon, just the regular one.
I tried everything suggested here with no success but thanks for the responses.
Only thing I can think of - after a 7 clean install the empty/full was normal. I've had 7077 since it was leaked, I've loaded and deleted a lot of programs, so something probably happened in the middle of that. Don't know what would cause the trash icon to only halfway work, or if it started at the time I first noticed it. So I did the oldest system restore point and STILL have the problem. (For some weird reason I prefer an always empty can to an always full one.)
Who knows.