w00t. For any who are interested, Opera and Chrome have the capability of having "user javascript" that runs before the website begins loading, which allows secure objects to prevent things like this. Nothing built-in for IE / Safari / Firefox, from what I can tell. Greasemonkey loads after the page is done, so it can't guarantee this sort of thing is blocked.

In Chrome (dev only, currently, and weird / lacking the ability to manage them after install), use a "// @run-at document-start" line in your script: http://dev.chromium.org/developers/design-documents/user-scr...

In Opera, it looks like it always runs them first: http://www.opera.com/browser/tutorials/userjs/

And the important piece, something which can block this kind of probe, Caja: http://code.google.com/p/google-caja/

Obviously other possibilities exist, and this will likely need tweaking, but it's a solid start. If nothing else, it reads as a fairly simple how-to to make your own leaner version if you just want to target this.