Last May '04 I took a glider ride with an instructor and no id was asked for -- caveat volatility of TSA rulemaking.
Of course great gobs of bureaucracy required if you want to fly solo on a US registered a/c.
Sadly the same regime has recently been imposed in Australia which forced an air safari outfit that catered to foreign pilots for many years without incident to go out of business
So far there's not any bureaucratic barriers to Canadians flying down in Canadian a/c.
Perhaps there's a business opportunity for G- and C- reg rental a/c in the US and OZ
