I have the same issue as the first guy, so I followed your instructions. I first changed the MTU to 1492, and it signed me in to PSN. When I tried to go to Playstation Store, it stayed on the loading screen for about 2 minutes before I cancelled. I restarted the PS3, but it timed out again and couldn't sign me in.
EDIT: Disregard the last comment. When I renewed my comp's IP address I could get on the store. Thanks for your advice.