Does a new spouse's insurance have to cover you if you have a pre-existing condition even if you currently have your own plan but plan to cancel it?

Yes, but you have to do it within 30 days of the marriage. This presumes you are talking about Employer Group Insurance. For more details see http://www.steveshorr.com/late.enrollee.htm

http://www.dol.gov/elaws/ebsa/health/11.asp