I'm seeing much better compatibility across the board now. Only one of my mobos (the Zotac with its horrible BIOS) can't get its interrupts routed properly without a line /boot/loader.conf to disable acpi's int routing.
The rest appear to work with APIC_IO enabled (which is our desired default), and the power button also now works. The monster 48-core server also boots cleanly now with no special requirements. -Matt