I wouldn't normally be posting this because frankly there are enough users of Reader out there who can post bug reports. However, I found a really strange workaround that I wanted to make sure was "out there" in case someone else in the community might benefit.
The situation is: Acobat Reader X locks up after Save As...
In my particular situation these were pretty large single-page PDFs, probably saved directly out of Photoshop or Illustrator. I had added all sorts of annotations using the Note Tool. After Save As... the Save dialog box didn't go away, even after half an hour.
However, just as I was about to give up and use Task Manager to force quit, I opened up Photoshop CS5. Instantly Reader unfroze and the file was saved and everyone was happy.
The next time the same situation occurred, I didn't even bother waiting. I just relaunched PS5 and it freed up Reader again.
So if you have CS5 installed and you run into Reader locking up, why not try launching one of the CS5 apps? Worked for me.