Hideo Kojima announced on his blog that there would be a new title in the Metal Gear Solid series on the PSP. Yes, Metal Gear Solid, not Metal Gear Acid. Gamers will play as Big Boss, as well as being able to control other members of Foxhound (whether that's within the main game or as an extra mode, I don't know), so the game must take place either between MGS3 and Metal Gear or between MG1 and MG2. Kojima mentioned that objectives in the game will have multiple solutions, and that it will support multiple players over WiFi.

Source: The Magic Box