I have encountered the IE6 CSS sprites problem myself - I blogged it here:
There's a really easy fix - I quote myself here...
Luckily, the solution wasn't too bad. Internet Explorers passim seem to ignore overflow:hidden when a child element is set to position:relative. Not good news... but the solution is easy - set the element with overflow:hidden to also be position:relative and then change the position:relative declaration on the child to be position:absolute... the jobs a goodun.
The full details can be found on that link.