I have a PS3 (2nd-hand 60Gb model, so I could play PS1 & PS2 games, and use the extra USB sockets)
I had it hooked-up with composite/Scart to a 32" SD TV at my old house, and now run it with a 32" 1080 HD Toshiba Regza.
When setting up the PS3 it will detect screen resolution and adjust accordingly but the HD really makes the PS3 gorgeous – text is easier to read and the games look how they were meant to, and obviously takes advantage of the BluRay feature.
In terms of future subscriptions etc for the PS3 gaming network – they will be coming.
http://www.sony.net/SonyInfo/IR/info/strategy/pdf/200911presen.pdf
Not a reason to NOT get one at all though : )