Although the Datel Blue Tool boots your PSP 300 into service mode, it's not a real service mode that let's you install and run homebrew. This latest TIFF Exploit, however, is one step closer.
The video above shows (turn the sound off if you're at work) shows developers "Davee" and "Bubbletune" using a TIFF exploit on 5.03 firmware, placing the PSP into the correct state that it can eventually run custom homebrew apps. It's not quite finished, but it should be close now. [Slashdot via Technabob]
Update: Nevermind! Looks like Chickhen, the homebrew enabler, is available now. Thanks karltpb.