Jessica Zahn, a Program Manager on Microsoft's eHome team, announced the following on thegreenbutton:

"People can continue using their V1 Media Center Extenders with an XP-based PC, but for those who plan to buy Windows Vista compatible extenders in the future, there will not be any rebate program for the V1 devices."

We had been hoping that some accomodation would be made for the early adopters of v1 technology, but sadly, that won't be the case.