OK, I've installed a brand new database version and switched the table type on atomic.users to be InnoDB. This should almost instantly fix a bunch of the locking issues because InnoDB has row level locking. The only downer is the possibility that InnoDB still has the bug we tickled a few versions ago when we tried last time. If it does, Heikki promised to fix it really quickly anyway :-)
Fingers crossed guys....
Steve.