"I had this same problem... Someone gave me their Google IO droid as a re-gift since they already had an Android phone and I already had a Verizon account. I was really surprised when I pressed activate and it just worked without any sort of prompt for information.
I initially called VZW customer service from my existing phone and was told I couldn't activate the free droid because it was assigned to another account. After much research turning up nothing, I decided I would just call customer service FROM the free phone, couldn't hurt, right?
Bingo! Worked. Here's what happened...
I pressed 0 to speak to an operator and was routed to "corporate customer service" (not exactly sure of the name, somehow different from consumer, anyway). I had to press random numbers for the SSN/Tax ID because I wasn't sure what to press since just "#" was not an option.
I told the rep what the deal was and she took the free Droid off the temporary account then transferred me to customer service where they activated the phone on my account. I'm glad I figured it out, but it would have been helpful if they were a bit clearer in the instructions with that.