I'm using Beta 5. I have tried modifying the key assignments both in my own defaults and the system defaults, and hit Apply. However, when I return to the editor the key assignment changes have not been retained. When I open up the key assignments screen again, the old assignments are still there.

Pending a fix for this issue, are the key assignments stored in a text/xml file somewhere that I could manually edit?

This should be fixed in Beta 6, which will be available later today.