I've seen the bug on Line spacing cosmetics but it doesn't seem to address the bug I'm seeing.
If you take a look at this bug, on my screen at least (Ubuntu 10.04 Firefox 3.6.10 1280x800), the h1 header has too small of a line-height and therefore overlaps itself.
bad line height http://img259.imageshack.us/img259/5185/badlineheight.png
The culprit appears to be
h1 { font-size: 170% }
.post-text { line-height: 17px }
Removing either of those fixes the problem (although obviously there are side effects). I think a simple override such as
.post-text h1 { line-height: 1em }
solves the problem nicely.