Here is the text of the message that they just sent me:
Dear valued cashback customer:
We are writing to notify you that the Bing cashback program will be discontinued, and the last day to earn cash back on your Bing Shopping purchases will be July 30, 2010.
Until July 30, 2010 9:00 pm PST, it's business as usual so continue to take advantage of great offers from your favorite merchants. You can redeem all of your earned cashback savings consistent with the cashback terms and conditions and access the Bing cashback customer support system through July 30, 2011. We encourage you to redeem your cashback savings and to further support redemption, we are waiving the $5 minimum payout effective July 31, 2010. To assist with prompt delivery of your cashback earnings, please visit http://cashbackaccount.bing.com to ensure your account information is current. For more details and answers to your questions, please visit our frequently asked questions page.
Thank you very much for being a loyal cashback user. We remain committed to delivering great value to our customers, and we are currently working on an exciting new program which you will hear more about from us later this summer.
Bing cashback team