If PS Cloud is a realtime mini-facebook for psn users, it would be huge. There would be a real niche for it. Id love to be able to see when my friend are unlocking trophies etc. It would create a great community experience.
Of course, it could be something completely different. My bets are it's a tie in with the xmb and home, like an update feed, that will tell you when one of your friends logs into home/playing bowling/unlocking a trophy etc and their update status, possible in game screenshots for others to see, facebook-style profiles . I'm only going on whats rumoured though.