UofT, you should try to submit your application as soon as possible. American schools have rolling admissions, meaning the sooner you apply, the better your chances. Even though you can't write the DAT until November, you should still submit your application as soon as you can. There have been reported instances (on SDN) of interview invitations even though DAT scores had yet to be submitted.
If you intend on applying only to American schools, then you could write the American DAT; however, if you intend on applying to Canadian schools, you should probably wait to write the Canadian DAT - Canadian schools won't accept the American DAT. As for diane's comment about schools not accepting the Canadian DAT, those schools also don't accept Canadians (or any non-US citizens).
Hope that helps.