Ugly security hole found in Greasemonkey 0.4. Thankfully, I'm still running 3.3. It's recommended that you uninstall and downgrade immediately. The bug in question does not affect Greasemonkey 0.3x.
This is a great example of why the open source model works so well, though. The hole was found by someone looking at the source, and now warnings can be sent and the hole can be closed before any harm is done. In a closed source model, bugs typically aren't found until something goes horribly wrong.