With the introduction of Reason 6, released September 30th, 2011, Propellerheads has developed official support for 64-bit ReWire as well.
 
Propellerheads has made the official statement that 64-bit ReWire must be supported in both your slave application (Reason 6, Reason Essentials) as well as your host application (SONAR in this case). You can find more info on Propellerhead's Reason 6 FAQ.
 
Prior to the SONAR X1d update, ReWire is only supported in 32-bit versions of SONAR. As with all new technology, implementation must be developed between the applications to work properly. The SONAR X1d update implements 64-bit Rewire support.
 
If you are a SONAR X1 Expanded, Producer, Studio or Essential owner, you can install the free SONAR X1d update to use 64-bit Rewire devices in 64-bit SONAR. This update can be found by clicking here.