From the FCCs site:
Licensing
On October 25, 1996, the FCC released a Report and Order in WT Docket No. 96-82 eliminating the individual licensing requirement for all aircraft, including scheduled air carriers, air taxis and general aviation aircraft operating domestically. This means that you do not need a license to operate a two-way VHF radio, radar, or emergency locator transmitter (ELT) aboard aircraft operating domestically. All other aircraft radio stations must be licensed by the FCC either individually or by fleet. Have been through Canada many times and don't think I've ever been asked for it. Having said that, I do have one, because I don't want to be stuck on stupid somewhere, having to explain why I didn't think a certain requirement applied to me...
You can do it online, and I'm willing to bet that like re-registering your vehicle online, you will get a confirmation of it immediately. In a pinch it must be better than nothing. You want form 605c which is the one for the aircraft, although if you're really excited you might get the one for the pilot as well. The license for the ship is now $110.
If memory serves me right, the license for the ship is good for ten years, and the license for the operator is good forever.
I might add that for traveling north I have a small file protector that has my passport, radio licenses, aircraft insurance info, past customs receipts, and all past gun registrations for Canada. It stays in a safe at home and is ready to go whenever I am. I have never been hassled on either side, I guess there is something to be said for having your stuff together when you show up...
Take care, Rob
http://wireless.fcc.gov/uls/index.htm?job=home