The only text editor I know of is in the PSP PDA homebrew application.
PSP Emulation News PSP Gaming PSP Hardware PSP Homebrew PSP hacking PSP Updates Playstation Portable PSP movies PSP consoles PSP Shop Buy Preoder PSP Here - Great application that can turn your PSP into a PDA, minus the touchpad. Everything you would want from a PDA is here, an address book, calender, planner, stopwatch, alarmclock, notepad, calculator, everything. It even has it's own p-sprint keyboard. It is a full file manager. Not to mention it's a media center, so you can also listen to your music and view your pictures and all that with this, can even create playlist. It even has theme support. This is a good app to have.
That's taken from Firedragon_King's homebrew guide stickied at the top, not sure if it's what your after though.
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Awful linkage
bit of a noob to this so dunno how to fix it, sorry!