Looks like this was released a week ago. Only will work for 8320 and 8820 over wifi and only on 1 plane so far.
From the engadet article:
"In a welcome first for domestic airlines, JetBlue will be rolling out free in-flight Yahoo IM and email services to passengers packing WiFi-equipped devices, starting aboard its new "BetaBlue" Airbus A320. Once this test-bed passenger jet reaches 10,000 feet, an in-plane network with three in-ceiling access points is activated, allowing most any wireless gadget with a Flash-enabled browser to view specialized versions of either Yahoo Messenger or Mail through a universal landing page. What's more, owners of certain BlackBerry handsets like the 8820 or Curve 8320 can keep feeding their addictions non-stop thanks to an agreement between JetBlue and RIM."
Rest of the article online: JetBlue tests inflight BlackBerry, email services - MarketWatch JetBlue introduces free in-flight email, IM - Engadget