Here's more detail about the PSP Go (drops 10/1/09):
Unlike previous PSP models, the PSP Go does not feature a UMD drive, but instead has 16GB of internal flash memory to store games, video, and other media. This can be extended by up to 32GB with the use of a Memory Stick Micro (M2) flash card. Unlike previous PSP models, the PSP Go's rechargable battery is not removable or replacable by the user. The unit is 43% lighter and 56% smaller than the original PSP-1000, and 16% lighter and 35% smaller than the PSP-3000. It has a 3.8" 480×272 LCD (compared to the larger 4.3" 480×272 pixel LCD on previous PSP models). The screen slides up to reveal the main controls. The overall shape and sliding mechanism are similar to that of Sony's mylo COM-2 internet device.