I have set up a sandbox project at http://drupal.org/node/1570130 which allows you to write separate cache keys to the database per domain. I would appreciate it if some of the domain access gurus could cast their eyes over it and let me know if I'm doing anything horribly wrong. Basically I'm creating a new cache backend which extends the DrupalDatabaseCache and prefixes the cache keys with the domain id. There is also a class there for memcache, but I haven't test that out yet. Anyway, I would appreciate any feedback you've got.