If anybody else has a persistent problem with as-daemon taking up 100% of the CPU whenever the net connection is up, regardless of reboots, *and* regardless of the fact that no MfE account seems to be configured (strange, that), I ended up doing

chmod a-x /usr/sbin/as-daemon

as root. Pretty drastic but sure "fixed" it for me.

As for the "no account configured" part, I *did* have sync configured previously. It subsequently stopped working and I removed the config via the control panel app. Yesterday I tried to reconfigure it but it kept hanging at the login stage, so I tried debugging it as per the MfE debug instructions. The strange part is that syslog shows as-daemon syncing some calendar events from my exchange account... even though it's not configured at this point.

Oh well. I'm just glad the runaway CPU hog is gone. I'll worry about syncing my work calendar another time.