Other World Computing (OWC) has announced 64GB 1066MHz DDR3 ECC
Memory Upgrade Kit for Mac Pro 8-Core Pro models.

The memory upgrade kits for Mac Pro 8-Core models offer more
configuration options and savings over factory memory upgrades,
according to the folks at OWC. The two 8GB module-based OWC upgrades
-- 48GB (6 x 8GB) and 64GB (8 x 8GB) -- offer a maximum performance
configuration for Mac Pro 8-Core machines not available from the
factory.

In addition to the savings offered by OWC Upgrades, Mac Pro users
also retain their original factory memory modules that can either be
set aside for future use or be sent to OWC for a cash back trade-in
of up to US$25 per Apple factory 2GB memory module. OWC offers a
guide to select and order OWC Memory Upgrade Kits for Mac Pro models.
You can find it here:
http://eshop.macsales.com/shop/memory/Mac-Pro-Memory