Some weird stuff is happening, I am converting an application that used to use javascript to open another web page in a tiny window for data input to use a ModalPopupExtende
I have successfully implemented a complete hack based on Jen's response. I use an asp:HiddenField to hold the value I want to post back, and I populate it with a pure HTML input box
This is lighter weight than Jen's solution as you're still only posting back one server control.
BTW, this is a very significant bug in the Ajax Toolkit. You can vote and comment on it here:
CodePlex Issue
Pretend you're in Chicago. Vote early, vote often.