According to a post on PS3 hacking/modding site PSX-Scene, Sony has managed to cut PlayStation Network access to jailbroken PS3s using older firmware. According to the site, PS3 owners using custom firmware had two methods for getting by the authentication process prior to Sony's fix. One method involved changing the DNS settings on the PS3 manually, then redirecting PSN requests to a web server hosting a hacked verification file. This would cause it to send a message to the system, telling it that it is running current firmware. The second method employed a similar process, but used a PC as a proxy server to intercept DNS requests. Both methods have - for the time being - been made ineffective.
For those running custom firmware on their PS3s, PSX-Scene offers the following advice:
"Anyway, back to PSN… Like any situation in life you have three options: 1) Buy a second PS3 (Sony will like you more), hack it or use it for PSN, and do the opposite with your current console; 2) Forget your current hacks and upgrade your console to firmware 3.56 right now; 3) Sit, chill, and wait for future hacks potentially getting you back on PSN … but you know the risks involved with that."