The models are mechanically and conceptually proven, and represent a very fertile base for further development, research, and teaching.
The last deadline for the program was January 2009, but NSF will, we understand, soon issue a special solicitation prompted by increased funding it received from the American Recovery and Reinvestment Act. See this NSF special notice, and an excerpt:
The Foundation also expects to expeditiously award funds as specified in the Recovery Act for: the Math and Science Partnership program (funded at $25 million) [several other programs mentioned] NSF will post a solicitation this spring for the Major Research Instrumentation Program (MRI) in order to make a sufficient number of awards to utilize the $300 million provided in the legislation. The Foundation currently anticipates that no other solicitations will be posted that are solely in response to the Recovery Act.
Here's a very fresh (April 2009) PDF presentation by Dr. Randy L. Phelps, the MRI Program Officer.
May 14 update: NSF has released the special MRI-R² solicitation.
May 28 update: NSF hosted a webcast on the MRI-R² and ARI-R² programs today; it's archived here.