A textbox on my form may change depending on what\'s selected in various drop down lists.
Is there a way to call a javascript function when the textbox value changes? <
I'd recommend to look over YUI's Custom events for Example: YUI Custom Event Example Link
All you need is to attach this script with Events manipulations
and then you are able to use Custom Events (everywhere in HTML) fnSubscriberChange - inside this Func you could do whatever you want. You could put your textbox change value logic here and depend on the Changed Select box value - call desired func. Logic that depends on Event listening is clear and easy to modify; YUI library supports all browsers so you shouldnt mind about browsers features.
Text box and select boxes:
Script for events: