I don't know about downloadable, but Radio Amateurs of Canada has an online lookup on their site. I suppose it's possible they would share their data (or its source) with you.
I stand corrected. At this link: http://www.callsign.ca/index_en.php There seems to be a "download" tab where you can get a Zip file containing a (poorly-formatted) text file containing a database dump...
Lindsay - did any of the links here work? It looks like some of them are broken, but I believe the link http://apc-cap.ic.gc.ca/datafiles/amateur.zip should be fairly reliable and kept up to date. When I downloaded it just now, it had today (Saturday)'s date on the top of the text file. I'm pretty sure it's the same poorly-formatted file Bezking mentioned.
Looks like it's a fixed-width file - the format is in the readme included in the zipfile.