Dynamic drop down lists are a long standing problem for QuickBase users. QuickBase itself discourages the use of dynamic drop downs and in many cases there is merit to their argument but when you need them you need them.
I've developed a method to manage dynamic drop down lists in QuickBase that involves wrapping a form in a frame and then intercepting changes to the fields which should affect the drop down(s). Drop downs are populated with JavaScript using QuickBase HTTP API calls similar to the QuickBase supplied Automobile Makes and Models example. My solution manages frame breakout and most back button issues.
The benefit is that the resulting entry form is fully integrated with your QuickBase application. It's also much easier to maintain than an XSLT based solution.
Please contact me if you've a QuickBase project that needs dynamic select boxes.