I just tested this out for myself. If you received a free copy of Vista from your participation in the beta program, the Ultimate key you were issued qualifies for the Windows Vista Family Discount. How's THAT for a discount!
Now, if only they could issue correct PID keys for the Home Premium installations...
Speaking of which, if you were issued a bad key, just install Vista without it. When it asks for the Product Key, just select "Next", then chose "Home Premium" as your edition, and continue as usual. Then, when Microsoft sends you the right key, you can add it to your system.