Im running it on a x86_64 box as we speak.  How can I tell if there is a
certain mode that is permitting this?

If the boot up says you're running with PMtimer then it uses the fallback
(usually happens on pre Fam10h AMD boxes). A typical Intel box
would use the faster ring 3 only TSC path and then explode with your
change I bet. 

Or step with gdb through gettimeofday() and see if it does a syscall.

-Andi