I'm using the darkfire theme upgraded from 1.2 to 1.5. The sidebar column is overlapping the main text. Check my site out using I.E. 6.0 and a PC and you'll see the problem. It only happens in IE on a PC. On a Mac on various browsers it works fine, and probably works well with firefox too.
I know IE has CSS pr0blems and I reviewed the forum but so far I could only fine references to problems with images in a post. Nothing with the default column setup. Never had this problem in WP 1.2.