At the beginning of the latest edition of the Kojima Productions podcast has confirmed that the development team behind Peace Walker (who also worked on MGS4) are now starting to work on MGS: Rising:
"Every once in a while we do this [move around]...as one project finishes, then the next project starts up, so the key people have to relocate. Peace Walker's winding down so Rising is winding up, so people have got to move around..."
Kojima also confirmed earlier this week via Twitter that he was working on a "Confidential Project," so it is unlikely that his entire team is working on Rising.
Peace Walker is set to come out on April 29th in Japan, with a North American release on June 8th, and a European release on June 17th. Metal Gear Solid: Rising still does not have a release date, but with key players from the MGS4 team now working on it, hopefully the development should speed up drastically.