No discount for
DVC, but if you pay for the Photopass CD in advance you can save $50 ($99 vs. $149). And if you don't get enough photos that you think makes it worth it, you can get a refund after your trip.
Here's the pre-purchase link:
http://www.disneyphotopass.com/previsitoffer.aspx
At a price of something like $12.95 for a 5x7, it doesn't take too many photos to equal the price of the pre-purchase CD.
Also, just for general information about the Photopass:
www.stitchkingdom.com/disneysphotopass