Mananation reports that the split was "amicable," but it seems very strange given that there are still several TCG products slated to be released in 2010, including class starter decks and the Wrathgate/Northrend sets. What this means for the future of the game is uncertain -- Blizzard might re-license the game to a different publisher or develop it internally, or pursue other means to keep the game (which was apparently doing well financially) alive. This also apparently affects the WoW Minis game, which might not have been doing as well.
Upper Deck has had some legal issues lately, which may be the root cause of this split, but we won't know for sure until we get a comment from either company. We've confirmed the split independently through Potomac Distribution, and we contacted Blizzard and Upper Deck Entertainment around 6pm EST requesting their comment, and have yet to hear back from them. We'll update the post if we get a response.
Update 7:56 p.m. EDT: Upper Deck replied to us letting us know that we'll hear from them on Monday concerning this matter.
Update 8:10 p.m. EDT: A Upper Deck representative on their forums posted the following about an event happening in Houston: "We're going to make Houston all kinds of awesome, and I'm confident this will be a great start to a new chapter for the World of Warcraft TCG."
Update 9:15 p.m. EDT: Zarhym hit the forums to confirm that the TCG is no longer in the hands of Upper Deck, but is not being discontinued entirely.