I read up some on the Push2TV thing and it seems that that will only mirror your SP and not do an extended desktop. Could be wrong, but since they never come out and say it will do extended, I assume it won't. Sucks cause that would have been uber-cool.
Still, I am really enjoying using my desktop as the main office machine and using SyncToy to keep my SP up to date. Much easier to take written notes on my SP when I'm on the phone if I can just hold it like a pad of paper without any cables hooked to it.
For me, any device that uses a pen really almost needs to be an accessory because you need the positioning of it to be the most flexible.