I got Cubase SX3 to rewire to Reason 6.5
I thought it would be worth adding this thread as there have been so many about problems with Cubase and I'd been struggling with it. Most posts refer to 32bit vs 64bit and that may be the answer for many people with newer kit, but in my case it was never likely to be the answer. Four year old laptop running Windows XP SP3. I found the solution by happy chance at the end of one of these threads and didn't believe it would work until I tried it, so thanks to that thread starter - something like samejjames?
Do a search for rewire.dll files. You should find one in program files\common files for your version of Reason. Select and Ctrl/C to copy it. Explore to your Cubase folder, probably under program files\steinberg\cubase... where there should be some .dll files. Copy the rewire file into there. Then open Cubase and Reason. Bingo! (well it worked for me..)