1. There is no recommended distribution. We have tested with the most basic configuration, but there is no reason to believe that it would not work with any distribution.
2. We have extensively tested with both Essex and Folsom.
We have not tested against Grizzly. Initial indications are that some changes will be required (although these should be fairly minor).
3. I am not aware of any, but I've heard that some may be starting soon (currently being evaluated). We would like to start collecting stories around these implementations.
4. OpenStack, EC2 and vCloud Director will be first class platform elements in IAC 4 and the MCAK accelerator will no longer be required to support these platforms. We are working on continuing to provide an MCAK framework and extension guide available to provide additional platform support (e.g., direct Hyper-V or other public offerings like Azure).
2. For IAC 4, I would assume that Grizzly and Havana support will be forthcoming. I know they have been testing with Grizzly and plan to get to Havana soon (if not already). Whether these will be in the first IAC 4 release, I will defer to the IAC team to comment. What I can say is that there is no effort with MCAK to support Grizzly/Havana with 3.1.1.
4. My understanding is that the GCOE lost funding for the project, even though they were near completion. I'm sure they would share their work if someone wanted to complete it.