I played Space Invaders Extreme back at Tokyo Game Show and I loved it. It has a trippy style and a frantic shmup feel. At that time Taito only had a DS version around, but a PSP version is in development too. The official Space Invaders Extreme page has a little blurb that it exists and it is scheduled to come out in February 2008, the same time as the DS version.
The PSP release has to be altered, at least a little bit, to adjust for the difference in vertical screen space. One part of Space Invaders Extreme had a huge Space Invader on the top screen while smaller Space Invaders crowded the bottom screen. Shrinking the sprites is one option, but hopefully Taito can think of a solution better than borders and eye-straining invaders.