During an interview (coming soon!) with Drew Murray, lead designer of the "spectacular" Resistance 3, we learned that the planned multiplayer beta is just around the corner. "We're doing a friends and family beta right now," Murray said. "And I think we kick off the public beta sometime in August."
That "sometime" has now been narrowed down to "the first week in August" according to IGN, reporting from the Resistance 3 panel at Comic-Con. The beta features multiplayer modes Team Deathmatch and Chain Reaction, both shown on video here, and an undisclosed number of maps. It's still not clear if owning the less-than-spectacular SOCOM 4 is the only way to access the beta.
These videos both take place on Seaside, so that's ostensibly one map we know about. And the first week in August is really a great time to spend by the sea; one last beach run before the game's September 6 release.