I have a site that's about to launch that runs both Radiant and Spree. I used a shop.domain.com subdirectory for the Spree store and the same graphic theme for both. It appears to be a single application to end- users, and the lack of shared authentication isn't a problem as having two databases for admins and designers is actually useful in compartmentalizing who works on what part of the site.
Cheers, Pete
