Hi guys, I've been searching everywhere on the net trying to find a solution for this problem. Everytime I started my PS3, it tried to connect to the Playstation Network, however, this always ended in an error (timeout). When I connected to PSN manually, it worked. Unfortunately, when I was not using the connection, most of the times it was disconnected again... But now, I think I have solved it!!! My PS3 has always been connected through LAN in a home network with a NAS, on which a media server is installed. I always thought that I should enable the 'UPNP'-setting on my PS3 (in network-settings), and I also enabled it on my router (Linksys WRT54G).
I thought I'd give it a try to DISABLE these settings both on my PS3 and on the router, and now the problem is solved!! I hope this solution helps many other people, since it has troubled me for months and months! This does not mean that de media server connection is disabled, you should still be able to use the media server connection.
Good luck, cheers!