That's basically the whole question. We need to be able to react -- preferably instantly -- in the event a Sharepoint workflow is canceled by a user. (Have to send some emails, update some database records, etc. Long story.)
How can we do that? I was thinking, for the lack of a better way, to create a timer job with a 1-minute interval that queries all of our list items, gets the associated workflows and checks their status. This would put a HUGE additional load on our servers, though, and an event-based solution would probably be best.