UPDATE: The sequel has been confirmed! There are two new games coming out called... Pokemon Black 2 and Pokemon White 2! Kyurem has 2 new forms and they are sequels rather than remakes.
Released June 2012 in Japan.
See my news post for more information.
The next Pokemon game would be released September 2012 in Japan (March 2013 everywhere else). The first games in every generation are always out for at least 2 years before the next one comes out.
It may be a third game in Unova aka "Pokemon Grey", but equally it could be Ruby/Sapphire remakes. Whichever one is not released will be out a year later. There is a Nintendo 3DS conference next week that may reveal some information (although this may just be the new add-on to the 3DS itself).