Update #5: Today skeezix has updated his PSP Atari ST Emulator to CaSTaway PSP Alpha v0.05. It now handles more than 250 disk images/savestates (up to about 1,000), includes an unpack demo, adds a scaling menu, and the throttle can now be set to 60Hz, 50Hz, 30Hz, or unthrottle mode.
Update #4: CaSTaway PSP Alpha v0.04 is now available, and it includes many fixes and also an ST Games database, new artwork, frameskip and throttling support among other changes detailed in the ReadMe file. A CaSTaway PSP Alpha v0.04b release also followed shortly today, and includes caching of database look-ups.
Update #2: CaSTaway PSP Alpha v0.03 was released today with LOTS of changes and updates (refer to the ReadMe for the complete list) including savestate and loadstate support along with a disk database. Shortly after, CaSTaway PSP Alpha v0.03c was released to fix a sound bug (menus disabling audio) and to add some control modes so you can work the joystick with the analog control. Nice!
Update: CaSTaway PSP Alpha v0.02 is now available with changes including the addition of a basic disk selector menu, full screen smoothed scaling, and an on-screen keyboard so you can navigate menus.
CaSTaway PSP Alpha, an Atari ST Emulator for PSP, was released by skeezix today. It is still in a very early (Alpha) stage, so although performance isn't great yet end-users can expect frequent updates the author states. For now, Triangle hits '1' on the ST keyboard, it can make it into some games such as Xenon 2: Megablast, and Right-trigger is 'Space' to get a few games going. Enjoy!