the laws that applied in 47 took your fathers citizenship away due to the parents taking out another citizenship.
Parliament has passed a new law, Bill C-37 that should restore his (and your) citizenship. It is retroactive to birth, from my understanding of it you are considered born to a Canadian parent.. We are in a group dubbed "lost Canadians" and the bill becomes active on april 17th 2009. If you are within this group, which I think you are, your citizenship is automatic and you don;t even have to apply. You would need to apply for proof of such (a citizenship certificate) But during that time, you are Canadian none the less. The fact that it is retroactive to birth allows dual citizenship. The USA only seems to bar dual when you obtain it as an adult (by choice)
I am not positive it applies to you, but from my reading of the law, it should be.
We have created a forum for all people who are affected by this bill. Please join us at:
http://lostcanadians.ning.com/forum
You may see more on the law and a list of who WILL and who WILL NOT become a Canadian under the law at
http://www.cic.gc.ca/english/citizenship/rules-citizenship.asp