The wedding would be this year (2012). Would a 'pre-nup" contract be valid in Philippines?

Yes it will as long as you got yourself witnesses from Philippine

A pre nup is a very wise decision if you intend on living in Canada. Divorce is not allowed in the PH, it's etched in frickin stone.I don't know how long you have known this woman, or what the age difference is, but I advise you to cover your every which way but loose.Have immigration run her name as well.
My friend was married in the PH in Nov 2011 and was dead 3 days later. All his bank accounts were cleaned out, assets transferred etc etc.
I hope you have known this person for a good long time , because the down side is really a downside. You might want to avail yourself of some of the Philippine marriage scam websites and make sure your eyes are wide open.