There's no crossing systems like that - if you have PS3 you can only login to the PS3 network, some with the Xbox.
I'm telling you right now, if you ever get an Xbox, then see the PS3 and their worthless online network, you'll be terribly thankful you went with the Xbox.
Xbox Live is totally kick ass dude. No doubt about it.
Nope thats what i have to do every time i want to get on line with an Xbox. With a PS3 i just sign in and i can get on there and play games online or surf the Internet.