CanonPhotog: Personally, I don't think a prize should be offered that can't be utilized be all participants of the Challenges. I realize that if the Challenge winner is not a Nikon or Canon user, that they can just sell the lens, but that kind of defeats the purpose of offering a specific product as a prize vs. just simply offering money as a prize to begin with. Certainly, it's nice to have third-party companies offer up prizes like this to DPR Challenge winners, so kudos to Tamron for their generous offer. But honestly, the fact that the prize being offered is only usable to a select group of Challenge participants is a bit exclusionary. :)
I understand the sentiment, but in this case it is Tamron sponsoring the prize, not DPReview. The prize does have real monetary value and can be sold by the winner for a tidy sum if the lens is not compatible with their gear.