i just restarted my mac a few times. i kept trying to disable messages, but on reboot, i'd be signed in again. finally, i was signed out, and, after repeated tries, was signed back in... and then everything worked (and continues to work). not sure what happens if i were to reboot (and am in no mood to find out).
EDIT: it's a nightmare, really. i rebooted (for a different reason), and am back to the 'imagent quit unexpectedly' window every few seconds, and messages not working correctly.
hoping today (tuesday 13th) brings a beta with a fix...!
EDIT EDIT: deleting the "com.apple.imagent.plist" file in my users preferences (and rebooting) seems to, at least, keep that endless error message from popping up...
EDIT EDIT EDIT: oops, spoke to soon; that pop-up error is back. but 2 hours later, am signed back in, and messages works on my mac.