This is not verified but worth a go i have already updated my Ps3 so cant test but a friend told me:
* Go onto the XMB on the start up screen.
* Go to Network Settings and select Internet Connection Settings.
* Click Custom > [any option] > [any option] > Manual.
* For you IP address, you may want to go to your router webpage to find out this remaining info, but for the primary DNS you need to put 67.202.81.137 which should trick the PS3 into thinking you're fully updated.
* Choose your settings from here on
Let me know if you can log into PSN i imagine if this works sony will make yet another line of code to stop this but im hearing mixed reviews that this works.