Since upgrading to 3.1.0, I have not had Solaris guest addition problems with a Sol 10 u8 64-bit guest on a 32-bit WinXP host.
However, I just copied my Solaris VM to a WinXP 64-bit host, and I am having terrible screen resizing problems, just like you. On the new host, my screen is 1280x1024, and I cannot get the guest to automatically resize automatically as I have before. On each logoff/logon, I have to host-f/host-g several times to get the screen to auto resize, which usually kills GNOME at least once. The resizing does not hold upon guest reboot.
I installed the 3.1.0 Solaris GA package when the VM was on the 32-bit host. Should I remove and re-install the GA on the new host?