Your comments

This is why allowing plugins to display a Web "page" via e.g. a simple WebView on OSX may be the best way to go: the underlying OS takes care of rendering UI widgets. I am sure Windows has a similar mechanism (I think it's called a WebBrowser control).
Yes, although e.g. a simple HTML view a la Textmate would be even more flexible (i.e. by allowing for buttons and other HTML forms elements). I have already made a similar comment on a request for a dialog API.
Yes! Yes! Yes! The quick panel API in ST1 is OK, but I always wished I could have a little more interaction for my Latex plugin, a la Cocoa Dialog in TextMate. In fact, HTML (forms) would be enough, for instance.