According to GameStop, Resistance 2 is set to release September 3rd, 2008 in the US for $59.99.
GameStop's release dates, as you well know, aren't always terribly accurate, especially this far off from the title's supposed launch. Their dates are usually estimates or dates sent in from the publishers (in this case SCEA), and only when delays or cancellations occur are dates moved, so just take this news with a grain of salt.
On another note, GameStop and EB Games are now taking reservations for Resistance 2. As of today, for only $5, gamers can reserve a copy of this epic sequel and ensure that one is available the day the game releases.
*Update: We called a few GameStop stores to confirm the release date and apparently it's September 3rd, not the 18th. Once again though, GameStop dates are only estimates from publishers and corporate, and are not official.