On second thought, I don't have enough facts to start pointing fingers at the toolkit since the toolkit works fine out of the box on a non-DNN site. I guess I should have a look in my own house first (my DNN Installation). I'm guessing it's a compatibility issue with DNN's skinning engine personally. Please let me know if you tend to agree.

I believe that this kind of behavior comes from the way the modal extender works is broken with the way DNN works. I don't exactly know what the modalpopup extender does, but I suppose it adds a javascript function that changes the display attribute of the popupcontrol whne the targetcontrol is clicked. Since it's all client, I suspect that it should be using the getElementByID function of the codument object. Now, DNN creates controls under it's own namespace so the function must not be finding the control at all.

If you remove the display:none; attribute from the style, then the modal popup kinda works. Kinda, because it's generating a page error in ie 7. but clicking on a linkbutton to reveal the popup will work. More investigation is required.