Just posting here on the off-chance that somebody else has experienced the same issue.
I'm running Cubase 7 (on Windows 7) and since installing Cubase 7 I haven't been able to rewire Reason 3.0 to Cubase... Reason will work perfectly as a stand-alone, but as soon as I launch Reason (with Cubase already open) I get an error message, telling me that Cubase has stopped working.
I have sought advice via Steinberg forums and spent numerous hours installing, uninstalling, reinstalling, etc.
Any clues, suggestions, etc. will be gratefully received.