According to David Lennon of Newsday, the agreement between Major League Baseball and Nippon Professional Baseball that formed the posting system has expired. Until a new agreement is made, Masahiro Tanaka won't be able to post and join MLB. Technically, he could simply retire from NPB and become free to sign with whatever team he wants, like Hideo Nomo and Alfonso Soriano had previously done.
As a Japanese citizen, Tanaka is unlikely to do something that would damage his connection to his home league, so he'll have to wait. It has been rumored that changes were on the way to the posting system, so a new agreement could include that there will be three winning bids, instead of just one. Tanaka is not just the latest phenom to come over from Japan, but these changes could make him an ambassador between America and Japan.
There are many teams that could potentially compete with the Yankees for Tanaka. The Rangers already won the bidding on Yu Darvish, the Mariners won Hisashi Iwakuma, the Red Sox have Koji Uehara and have previously won Daisuke Matsuzaka, and the Orioles have previously signed Tsuyoshi Wada and Wei-Yin Chen. Teams like the Athletics, Mets, Blue Jays and Cubs could also throw in their hats.