I did enable ACPI and VT-x/AMD-V, but not IO APIC when I created and installed the Solaris guest.
Unfortunately, the extensions were disabled in the BIOS and enabling those didn't fix the problem.
I had read elsewhere that the options would be uncheckable in VB if the BIOS didn't support them, but I guess that's not the case.
Just enabling the extensions in the BIOS and restarting the virtual machine did not cause Solaris 10 to boot in 64-bit mode. So, I decided to try a new install with all the extentions on and more RAM to make things faster.
I ran into a tangle creating a new machine because I had allocated 2048 MB of RAM to the machine and it was causing the installer to crash. I thought it was all of the extentions I turned on and had to enable and disable them one-by-one to figure out what was causing the crashes
In the end, a new install with ACPI, IO APCI, and VT-x/AMD-V enabled in both the BIOS and VB did not get Solaris to boot into 64-bit mode.
Any more suggestions? Thanks for your help.