oh derp i completely forgot about this. i'll walk downstairs now and give it a test, and i'll edit this post when im done.
EDIT#1:
ok so from what i can tell, you can copy saved data locally between PS3 accounts(local system accounts) as much as you want, although depending on the game and its features, some games may not work if the account uses a different PSN(or at least thats what the thing told me when i tried to overwrite my own skyrim data with somebody elses data - although it didnt seem to complain on my alternate PSN which had no saved data at all)
using the same PSN and a different local account name i was able to just jump right in once i put the game in(updates were already downloaded on that console as i had done that before - you would probably have to re-download them.) so it doesnt seem to matter what account name you use.
also keep in mind that if your game has any online features at all, if you are not using the same PSN, those will probably be reset, so for example if you play call of duty, you'll get to keep your single player progress and what not, but you will lose all of your multiplayer rank progress and unlocks. and you probably will lose access to all your DLC and downloaded games as well.
so because i only play skyrim on xbox now anyways, i'll boot up skyrim and tell you how it works on my alt, on my main account, and on some brand new local account i just made which doesnt even have a PSN yet.
EDIT#2:
ok so on my alt account it let me load up the game, but saving and trophies were disabled. im assuming it will be exactly the same on my main account. but i will check on the brand new account i have made. make a note that this was done with their latest autosave so that might have something to do with it. i'll check with a real save.
EDIT#3:
ok so it doesnt seem to matter with a real save either, saving is locked and trophies are disabled. i also ran into the same situation on my brand new account which has no PSN at all. i've also run into this problem just copying files locally between accounts.
You should know however, that not all games seem dependant on this - for example in all cases i was able to load up my save of hot shots golf out of bounds with no complaints or restrictions. so it seems to depend on the game. some games will load, play and save, others will disable trophies, others will disable saving, others will disable both, and some simply wont load the save file at all. also for some games(like the original SFIV i believe) the save file will be locked to that particular console, so there is no way of copying it.
So in the end, some games you can play with a different PSN, but in most cases it is best to have the same PSN because then you can play absolutely everything(except for the rare case where a game is locked to a console, which is pretty much never). it doesnt matter if you're on a different PS3 with a different local account name, so long as you are on the same PSN.
also note that if you have a downloaded game that you download onto a console using your PSN, it seems everybody else on that console can play it including you, without having to buy the game again. just keep in mind that i believe you only get X number of downloads on these things, before you have to buy the content again.