SImple selectors based on class names (like $('.myClass')) fail in IE 5.5. I put up a test page here, with examples:

http://dev.sowsinsk.com/jquery/ie5-selectors/

It appears that when you attach a class name to an element in your selector (like "li.myClass") then it works... if you have the class name alone (like ".myClass") then it doesn't work.

I know that 5.5 compatibility is not the most pressing issue (jquery.com says it's supported, but the doc wiki says it has "known issues"). I think that it would be useful to either have these issues corrected, or to maintain a list of known issues with different browser versions on the wiki. It's useful to someone like me who needs to think abouty IE 5.5 compatibility at work (as unfortunate as it may be), and I think such a resource would be useful if they can't in fact be solved in the source.