Though still in the review queue on mozilla update, 0.5.3 is available for download directly from userscripts.org right now.
This update solves the problems with migration from 0.3.5 that some people where having with 0.5.1. It also fixes the "Install User Script" menu item always available bug that pissed a lot of people off.
I'm still working on improving the injection system, which will solve all the random injection problems people are having. This will show up in Greasemonkey 0.6.