I was just thinking, and I may have come across a way to lock a PSP, but I don't know if it's possible (I'm no coder).
Couldn't someone make an eboot, that when run, doesn't allow you to exit back to the XMB. Or if that can't be done, one that makes your PSP purposely crash when you try and exit to the XMB. When it's made, throw it into the eboot autorun. This would keep kids who didn't know about the recovery menu from accessing it. And when you want on, simply disable the autorun.
This would be very helpful to me, as there are several kids around my house, who who don't how to leave things alone, and find my memory cards when I hide them. Since I can't carry my cards with me at work, for fear of breaking them, I'm a little out of options.