Originally Posted by
swag
Airspace is irrelevant. A nonstop flight from Canada to the Caribbean over US airspace is no different than a nonstop flight from Canada that just flies over open international waters. You won't interact with US TSA in any way.
There is a small difference that most passengers would never notice. If the flight traverses the airspace of the 48 contiguous United States, then the passenger list must be uploaded to Homeland Security and screened against the various watchlists prior to departure, even if the flight doesn't stop in the US. So, if you happen to be on a US watchlist, you won't be allowed to board a flight that overflies but does not stop in the US. Plus the US gov't will have the details of your travel plans.
Speaking to the OP, don't let TSA scare stories hold you back. Air travel is as popular as ever: 2011, the latest year with figures available, set an all-time record for air travel in the US. If airport security were as onerous as some people claim, the number would be going down, not up.