Hi,

I assume that following could be cause of your issue:

http://stackoverflow.com/questions/2686362/fancybox-asp-net-button-not-working

Therefore, I would recommend you to use IFRAME option:

http://fancybox.net/howto

You can create simple page just with logon form web part (nothing else) in your content tree in CMS Desk and then load such page into fancybox IFRAME option.

Then it should work just fine.

Best regards,
Miroslav Remias.

Instead of the fancy box, you could also the jQueryUI dialog. That way you can just use a div in the page to define the contents of the dialog and then call the Dialog to display it. I am creating it right now to display a popup window to first time users of our site alerting them to the fact it is under construction. Check it out here.

I thought of using the FancyBox solution, but I decided to go with JQueryUI to keep it close to the original JQuery library.