This KB article has just been published:

Virtual Machines may unexpectedly reboot after a VMotion migration to an ESX 3.5 Update 3 Host OR after a Power On operation on an ESX 3.5 Update 3 Host, when VMware HA feature with Virtual Machine Monitoring is active.

There’s a work around for the problem but I will not be posting them here cause they might change somewhere in time. Just read the KB article for more info on how to fix this issue.