I'd like to present you my first C coded homebrew for PSP (used to dev in lua until I finally figured out how to install cygwin)
It allows you to protect your PSP with a password.
To install it, run the PBP inclued with your PSP (to set the password) and then put the prx (password.prx) in ms:0/seplugins and activate it via the recovery menu...
And now it will ask you a password at each XMB boot.
You can change the password with launching the EBOOT.PBP which you ran to install the pass...
To know more about it and check out a screen, french speakers can visit my website,clicking here
UPDATE: V1.1 : CORRECTION: the letters were too fast, not anymore!