From a Sony Press Release today:
"Sony Computer Entertainment America to Offer Downloadable Game Content for SOCOM 3: U.S. Navy SEALs Exclusively on PlayStation(R)2 Computer Entertainment System
Downloadable Map Packs Expand Playability of Number One Online Console Game With Both Old Favorites and Brand New Maps Playable Online for SOCOM 3: U.S. Navy SEALs."
Sony will be offering three different download methods for obtaining these maps. You can use a USB Flash Drive, the PS2 HDD, or even a PSP equipped with a Memory Stick 32 MB or bigger.
The first of at least three map packs will be available sometime in March and feature several classic SOCOM II maps including After Hourse, Last Bastion, and Liberation. They will feature tweaks and upgrades that take advantage of the new gameplay elements presented in SOCOM III.
It is currently unknown when the other two map packs will land, but one will feature classic levels from the first SOCOM and the other will introduce completely new maps to the series.