Well that's weird! I tried disabling my card again (a 1010LT) and I was able to shut down! Maybe some other random thing I ended up disabling was also hanging the shutdown. Here's what I'm trying right now based on a couple recent suggestions on the M-Audio forums:
1. Reinstalling current (Vista64) driver.
If that doesn't work
2. There's a Win7 beta driver. I'll give that a shot.
I'll give an update in about 30 minutes once I know whether either option works.
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UPDATE (and my answer):
Option 1 didn't work.
Option 2 DID work. I downloaded & installed the beta driver from here:
http://forums.m-audio.com/showthread.php?t=11130 -- you have to register to download, but it's quick & free
I did a cursory check in Sonar and it seems to be working just fine. I can shut down & restart Windows. Yay! Jackisback, let me know if that works for you.