I can report exactly the same problems here: Running multiple vms and trying to shutdown one freezes the process without beeing able to kill it via task manager. This happens with all types of guest OS I've tried: W2k3 server standard, Windows XP, Ubuntu 8.10. I didn't have this kind of trouble running Virtualbox 2.1
My configuration:
Double AMD Opteron 275
Windows 2008 SP1 64 bit
16 GByte RAM
Virtualbox 2.2.4
Regards,
Christian