Well, Geohot promised a workaround for Sony's removal of the "install other OS" feature in PS3 firmware 3.21 and now Geohot has delivered. While it's not available to the public just yet, Geohot's 3.21OO custom firmware will apparently be simple to install and, as you can see in the video after the break, it works just as you'd expect and simply restores the "other OS" option to its previous place. Geohot even says that the custom firmware might actually enable the other OS feature on the PS3 Slim, but he hasn't yet had a chance to try it out.
Engadget : http://www.engadget.com/2010/04/07/geohot-brings-back-other-os-support-with-ps3-3-21oo-custom-firmw/
Youtube : PS3 3.21 with OtherOS : http://www.youtube.com/watch?v=1-9wLWQ4-uA