Kingston, Crucial and many others are announcing compatible memory kits these days. Here's the latest qualified memory list. Hopefully this translates to more memory options for MP's as well. There are currently two vendors listed (Hynix and Samsung) with 4GB ECC sticks.
http://www.intel.com/technology/memory/ddr/valid/ddr3_UDIMM_RDIMM_results.htm
Kingston press release...
http://www.kingston.com/press/2009/memory/03b.asp
http://www.intel.com/technology/memory/ddr/valid/ddr3_UDIMM_RDIMM_results.htm
Kingston press release...
http://www.kingston.com/press/2009/memory/03b.asp