My machine also froze with this. With "hw.madt_probe_test=1" in /boot/loader.conf I get the "APIC_IO: Cannot route 8254 interrupt to CPU" panic. I have both options SMP and APIC_IO enabled, normally without any issue. Haven't tried disabling APIC_IO. It's a Core2 Quad Q6600 on an Asus P5K SE Mobo. Cheers, Nuno