Javaranch Tip: If you're going to post more than a line or two of your code, wrap it in UBB Code tags (there is a button on the edit screen to assist with this). They preserve your indening which makes the code easier to read. This usually results in more people reading it an helping you.
A hidden iframe is a way to do the same thing, only it's a bit hackish and ugly. Once the first list is changed, a hidden iframe is submitted. Once it returns with the data, it's onload function can be used to populate the second list.