Ok, so I got my extender working on a default.aspx page on my website and it looks good. I basically copied and pasted the code for it into a user control control.ascx page. When I do this I completely loose the functionality (just shows the target control label and no dropdown, even upon hover). Is there any reason why it doesn't work in a custom user control inside a masterpage setup?
Edit: Didn't quite do the trick. Any other suggestions? Its in a master page setup, using eo web tabs (I tried it inside the tabs and outside the tabs but on the same page as the tabs, to no avail), and its in a custom user control. Think there are dependency issues?
I don't know if this helps, but I had the same problem with the autocomplete extender and determined that the server-side function could not be in the user control, but needed to be on the page (or in a webservice, I guess). Once I moved the function, it worked fine.