I've continued to experiment with this problem (since build 2.2.4) with each of the VBox versions up through the current 3.0.4 and have found that I don't get the VirtualBox hang after Win XP Guest shutdown when "VT-x/AMD-v" acceleration is disabled (thanks for this suggestion Rpmurray!). Here are my observations to date:
1. An intermittent problem that seems to happen only after WinXP SP3 guest has been running for quite a while (multiple hour or days long sessions).
2. This started after several weeks of use of VBox 2.2.4.
3. Doesn't seem to be related to the use of any particular software on Host or Guest.
4. This hang requires force quit of VirtualBox and restart of Mac to get VBox running again.
5. Occurs when VT-x acceleration is enabled for WinXP Guest on an iMac Host with 2.16 GHz Intel Core 2 Duo & 2 GB RAM but NOT
on an iMac Host with 2.4 GHz Intel Core 2 Duo with 2 GB RAM (this Host has never shown this problem).
I found some other reports of similar problems in Bugtracker Ticket #4593 (http://www.virtualbox.org/ticket/4593
) for users with Windows Hosts. For now this a forum update for others that might be seeing a similar problem. Any advice with fixing it is welcome.