Captain M. - yeah, I too instructed in the USA with no FAA CFI rating.
FSDO (and the school) assured us that provided we didn't instruct to wards any FAA ratings - or signed off any student flight that required authorisation (see DesiPilot posting above) - we were allowed to instruct with just our FAA PPL (issued on the strength of...) and our UK instructor ratings.
I was never happy with this...but FSDO and the (reputable) school were. I'm not saying their view is correct...I'm just saying that the FAA didn't mind the situation provided we did not train towards an FAA rating.
Indeed, I was interviewed by the local office some years later, following an incident, and they agreed with the scenario..quite happy, no action taken, no file opened.
I still disagree with it - from a personal point of view....I think you should have FAA ratings to teach in N reg aircraft in Federal Airspace (but I'm not the FAA [just work with them]).
TAS!! Although I didn't make it clear, my comment on being snapped up by a US school at the moment was directed towards anyone holding a JAA instructor qualification. If you have (just?) Canadian or FAA then I would think, post 9/11, that your situation would be harder to justify.