Jon Webb, of the SCEA Developer Support Group, spoke at GDC 2009 about the PlayStation Network. Some interesting facts emerged about the network, detailing the nuts and bolts of the service as well as improments that have been added since launch. One of the mentioned features is multiple PSN sign-ins, which allows a second player to join a local multiplayer game using their own PSN ID. So far LittleBigPlanet and Resistance 2 (using patch 1.50) are the only games that allow this.
Webb mentioned that this feature will be added to other high profile games, though no titles were mentioned. The multiple sign-in feature will pull saved data from both accounts but making this work will be the responsibility of the developers of the games. Like LittleBigPlanet, any progress made in the game is saved to all PSN accounts used.