it isn't because they don't have enough room..
That did seem to be the case, when I floated the sidebar right and removed the large left margin I had to adjust the width to get it in there.
<font color='Blue'>width: 460px;</font>/<font color='Red'>*was 480px*/</font>
There may have been something that was adding to the width but I did not investigate further.
Your #content div had a width of 720px so there should have been enough room.
Yes, it must have been the default margin/padding on the ul. I removed 20px from the #left width and 20px is usually the default indent on a ul.
Safari was the only one I saw dropping the column down.