Well that didn't take long. Kotaku has posted a video showing the PlayStation Portable's 2.71 firmware has been successfully hacked. You'll see in the video how the modified machine runs games without the UMD disc in the drive. According to Kotaku, the hacked firmware can run both homebrewed games and ISO (disc images made of legitimate games) without a problem, though the article didn't specify how, exactly, to obtain said hack.
Knowing Sony, PSP Firmware 2.72 (or 2.8, depending on how important they choose to make the update look) will be out be week's end. Of course, they could just be nice and let us have our fun, but somehow I doubt that'll happen.
PSP 2.71 Firmware Finally Hacked [Kotaku]