There have been some questions about how pop-up blocking works in XPSP2 in the comments.
Jeff Davis, one of the IE developers who worked on this feature in XPSP2, has put together a post (http://blogs.msdn.com/jeffdav/archive/2004/08/25/220737.aspx) that gives more details on what we consider a user initiated click. If you want to get better sense for how it works, please take a read through his latest entry - mouseDown + mouseUp = click.
Thanks.Scott