I had the same problem in a 11.10 version of Ubuntu. As far as I remember it occured after accidentally deleting one of the system fonts from /usr/share/fonts . I upgraded to 12.04 in the meantime but the problem persists. I noticed that installing some fonts fixes the problem on some pages but not on all of them (can remember exactly now but I think that after installing mscore-fonts a dreamhost page started displaying correctlly) . I suspect that the default browser serif or sans-serif might be missing or sth...
in my case also sometimes fonts are too big for a text field or underline is crossing them out instead.
I managed to confirm that problem is caused by badly installed or missing fonts. I repaired display on github page deactivating my Helvetica font using 'font manager' (you can download it from software center), and it started rendering Arial instead. It seems like Helvetica installed on my system had wrong spacing or sth.
You can check the fonts used on websites using Mozilla firebug plugin, and 'inspect element' css styles 'font-family'.
hope it helps