By agreement between FAA and Transport Canada, any US certified experimental aircraft must have this Canadian Validation letter onboard for any flight in Canadian airspace. This includes overflights, such as routes from northern Ohio to the north Detroit area. The letter does not require signature, filing, or even notation of your tail number. It simply needs to be onboard. I suggest you print it, read it once, and keep it in your permanent onboard documentation folder (with your inspection/airworthiness/flight limitations letter).
http://www.faa.gov/aircraft/gen_av/ultralights/sfa/media/tcauth.pdf
Over the years, my firm has never encountered an enforcement action specific to an overflight without this letter. But, its absence can result in a denial of landing rights for a trip into Canada. And possibly fines.
You might never need it -- but it free and easy to get and keep a copy. And, under present regulations, it does not expire.