I bring my race bikes in from Nova Scotia thru New Brunswick, and back again with no problem. But I have stopped at the Canadian Border offices and got a "green card" which has either the frame number or the motor number written in and stamped by the Canadian Border officials, before I enter. This helps on the return trip, but quite frankly, I've never been hassled.
Theoretically, I've been told by one U.S. Border Patrol that I should have the bike bonded before bringing in into the U.S., which I know the boys coming from England have to do to bring their bikes to Bonneville, unless it's a titled and road registered bike. Granted, times have changed a lot in the last year. Best to have some type of positive ownership information.
Tom