I've been unable to get on to the PSN today.
It seems that I am not alone.
Anyone with an old style PS3 (ie NOT slim version) will have the same problem.
You will notice that your clock has reset to 31/12 and you will have lost trophies, some dynamic wallpapers and perhaps saved game data; and even downloaded games may be corrupted.
This looks like a SERIOUS problem for Sony.
The advice is don't even try to play any games. I have switched mine off and I won't be switching on until Sony have sorted the necessary firmware update etc....
More here: http://www.guardian.co.uk/technology/gamesblog/2010/mar/01/games-sony