I also want to downgrade using my N900, but I'm having a hard time getting my 60GB Fat with 3.55 in service mode.
I'v got the latest PSFreedom from extras-devel.
Connect my N900 to the right USB-port on my PS3 (there are four on the front and I choose the one that's closest to the blu-ray drive).
On my N900 I start PSFreedom, select JIG-mode and start PL3 Jailbreak.
The led turns red, now I completely turn off my PS3 with power switch on the back.
Then I turn on the PS3 with the power switch on the back and I press the power-button quickly followed by the eject-button.
Nothing happens, the led will stay red and the N900 will prompt for mass-storage or pc-suite mode when the PS3 starts.
Then the PS3 enters the main-menu on the dashboard...
No service mode on the fat ps3?