... but I will.
1) joshuaip, as a visitor to your site, I wouldn't see your pop-up unless I'd explicitly allowed it. Nor would most users of a non-Microsoft browser. When the next version (or service pack) of Internet Explorer comes out, neither will most Microsoft users. So, there's not a lot of point wasting your time on this now.
2) A number of surveys have been done that show users hate popups. We're talking 90%+ here, and some users will avoid your site as a consequence. A mac site [lowendmac.com] came up first when I searched about this topic, but it looks pretty typical.
YouDonkey.com [youdonkey.com] has an article about 10 Easy Steps to lose your customers [youdonkey.com], which repeats common knowledge but is still worth a look. Ironically, the page contains some scripting that prevented me from using a mouse-gesture to navigate back to Google. They just lost my custom, and I only looked at one page.
To summarize, I think it's a bad idea, though others may beg to differ. Provide a link that people can click to subscribe -- even with a pop-up window (onclick) if you like -- but don't force it onto users.