Also from InternetWeek:

The bug could let a malicious Web site read any local file on a Greasemonkey user's machine, or view the contents of all local drive directories, said Aaron Boodman, Greasemonkey's creator, on his blog.

"I'm working feverishly on a fix for this," said Boodman. "But [it] will take several days. In the meantime, I strongly recommend that everyone either install Greasemonkey 0.3.5, or else disable or uninstall Greasemonkey completely."