Per the official site:
Manchester City Football Club and NIKE Inc. today announced a new partnership that will see the sponsorship of the Club move from Umbro to the Nike brand from the start of the 2013/14 season.
The Umbro sponsorship, which has been in place since June 2009, will continue to the end of the 2012/13 season, after which all kit, training wear and related products will be designed, manufactured and distributed by Nike. The contract will run until 2019.
When City returned to Umbro ahead of the 2009/10 season it appeared that it was a long term move for both parties, but given that Nike own Umbro they were keen to capitalise on City's growing global presence.
UPDATE: Reports in the press this morning are suggesting that the deal is worth approximately £72million (£12million per season) over the six-year period.