Worth it on 32-bit.  On AMD64, probably not.  On Intel 64-bit, maybe, 
but less important than in P4 days.

Well most of Intel 64bit is P4 -- and Intel is still shipping
millions more of them each quarter.

This could change character completely if used at the tail of a function 
where you now have

sti; 1: ret

Which generates an interrupt holdoff on the ret, an unusual thing to do.

Unusual yes, but I don't see how it should cause problems. Or do you
have anything specific in mind? 

Worse is probably that on K8 this case might cause a pipeline stall
if there isn't a prefix in front of the ret.

-Andi