A somewhat decent way to determine the number of ACTIVE psn accounts linked directly to the ps3 would be to take a hit game/popular online heavy game and multiply their peak member count online and multiple that by that game's attach rate. If you do this with a nice sampling of games and find an average you will have a close idea of ACTIVE PS3 PSN accounts. Anyone up for the challenge?
Edit: I understand this can gimp numbers by leaving people out who use the PSN Store for movies but any better ideas?