Craig,
I had a similar issue when i unwittigly added a second css element named highlight. without properly checking thro', i had added a duplicate css attribute name to the default.css sheet. all was fine until i used any of the dropdowns. when i used these, a postback was occurring. of course, i didn't realise at the time that it was due to the css as i hadn't been aware of the duplicate naming. it took me literally a day to discover the error (i only discovered it when i tried changing the theme and noticed the issue had gone away).
{edit}
see post:
http://p2p.wrox.com/topic.asp?TOPIC_ID=58434
for the details of that 'error'
just goes to show, that not all errors are obvious. of course, my error in that case was plain stupidity (at not checking for the .highlight name being in use already) but certainly it means that i now thoughroughly regression test any css changes/additions in the same way i do 'code' changes.
jimi
http://www.jamestollan.com