Upgrading memory in Mac Pros is a pricy business if done directly through Apple. Folks with enough faith in their memory upgrading skills may decide that enough is enough and upgrade the memory themselves. For this reason, a number of vendors have introduced memory geared for use with the new Mac Pros, among which we find Taiwan-based Panram.
The new memory modules we find from Panram are called the Panram Mac Modules, and they will be available in speeds up to DDR3-1866 MHz. The DIMMs come with ECC support, so while it'll cost you extra, you can be sure that the memory will not only be stable, but also fully compatible with the Mac Pros. Timings for the 4 GB and 8 GB modules are set at CL11 speeds, while the 16 GB DIMMs will have CL13 timings.
Modules will also be built with DDR3-1600 MHz speeds, though these are not guaranteed to be compatible with Mac Pros. These DIMMs will also be available in 4 GB, 8 GB, and 16 GB capacities, all of which have CL9 timings.
The company also sells kits with the memory modules, with the biggest kit featuring a 32 GB capacity spread over two DIMMs.
Panram has listed pricing as "Open Price" on its website, so it'll be difficult to know whether you'll be saving cash by taking this as an upgrade route.