That didn't take long. The first Custom Firmware has been released.
http://www.ps3-hacks.com/2011/01/04/...gs-to-the-xmb/
The only thing it adds is the option to install homebrew packages to the XMB. There are currently no properly signed homebrew packages available yet, so this Custom firmware is more a proof of concept at this point.
I decided to test it out myself. Installing it was as easy as putting the file on a USB stick and choosing the "Update from storage media" option in the PS3 menu. The system booted just fine and everything worked. The new options were under the game menu. They didn't do anything however as I had no packages to install.
I decided to go back to the official firmware, since this modification really didn't do anything. I can't wait to see what they eventually come up with.