Team M33 has done it again. The custom firmware is out for new slim PSPs. The full list of instructions can be found, along with software downloads at:
http://www.dcemu.co.uk/vbulletin/showthread.php?t=75006
Following the procedure, you’ll create a Pandora’s Battery, a Pandora’s Memory Stick, then use them to program the new firmware. I have done it with my silver slim, and running my homebrew apps just fine