Jeff has updated his Atari ST emulator port CaSTaway for the PSP. In a short period there have been two releases, the latest one is v0.03c.

CaSTaway/PSP 003c alpha!
Fixed a sound bug in 003 (menus disabling audio), and added some control modes so you can work joystick with the analog control. (Included PSP 1.50 only; grab 003 normal for PSP 1.00)

CaSTaway/PSP 003 release!
Added all the fixins’! Savestate and loadstates, disk database, you name it.. its almost all in there! (Though still lots of finesse to add for next release, a little ways off.) (Includes versoin for PSP1.00 and PSP1.50)