PS3 Developer White Lord of DaTeHaCKs's Team has fixed the Gran Turismo 5: 2.11 update for 3.55 Users. Check below for the release quote by White Lord and instructions how to install this update on your 3.55 console.
Back with online again on 3.55 for the best PS3 Racing game. I did this for you playas to enjoy. Don't ride like mad...
Extract the content to USB hard drive or device. DO NOT RENAME THE FILE!!! Use Multiman 04.05...above version (update to latest version if necessary) Open File Manager (mmOS) in Multiman Settings Menu and browse the file. Double click it and confirm with Yes. Enjoy Online GT5 latest update on your PS3.
Also this was posted on IRC from gLacK for those interested: