Different modified firmware v3.41 floating around for downgrade. Be very careful

Developer Mathieulh confirmed that there are different modified firmware v3.41 PUP files floating around the web. Downgrading using an incorrectly modified firmware v3.41 will cause your PS3 to not be able to playback Blu-ray movies. According to Mathieulh, the modified v3.41 firmware that is okay to use must have the following SHA1 value: F3C19E06C0E7B8CC550BB3244F5F88356173FA6D.You can use MD5 & SHA-1 Checksum Utility to verify the copy that you have or simply download below, as it has already been verified to have the correct SHA1 value.

Sony rolled out firmware v3.55. Puts an end to downgrading

Sony just released firmware v3.55 for the PS3, which only comes with a security patch--essentially blocking the current downgrading method that was possible on v3.50. Upgrading to this firmware will block your ability to jailbreak and/or downgrade your system. So, DON'T DO IT!

Note: Greg, from ps3-hacks said that you can still access the Service/Factory Mode using the E3 Card Reader USB device, but the downgrading option is blocked when used with PS3 firmware v3.55. Ouch!