Was thinking about this myself the other day, as I do it all the time with friends that are on the Xbox Live Network. (via my own XBL account)
I tried signing up to the PSN network online, which was OK, but to add friends, you have to do it through a PS3, so the next time I'm at a friends house with a PS3, I was going to set up a user name via their machine, and see how that goes.
Hope that helps!
Live a life you love, use a god you trust, and don't take it all too seriously

Actually its brilliant ps3-ps3 just that you cannot access ps3 network from a pc, but time will tell