Via Scoble, here's word on what looks like a fatal flaw in GreaseMonkey:

From the mozdev mailing list:

Uninstall Greasemonkey altogether. At this point, I don't trust having it on my computer at all. I would think that whoever is in charge of addons.mozilla.org should immediately remove the Greasemonkey XPI and post a large warning in its place advising people to uninstall it.

By the way, "Greasemonkey Hacks" is DEAD until we fix this. And I'm posting a big red blinking warning on every page of diveintogreasemonkey.org advising visitors to uninstall it, until all of these security holes are closed. This is why God invented the <blink> tag.

Sounds pretty bad.