Ok hopefully someone will be able to help here
on my apple account i have my mac, iphone 4, ipad and the wife's touch
Now if a friend with an iphone sends me an i message will it go to the iphone ipad and the wife's touch? if so that ain't great! likewise he has his GF on his apple id so if i sent a reply would it go to both?
it's like the facetime issue i have, if someone facetime's my apple id it rings on my mac, but use my phone number and it'll go to my phone.
Interestingly though call my mac (email) from my mobile and i get a completely different mobile number displayed than mine, dialling from the mac to the iphone using my standard number won't go through but use this mystical number it does -

I hope that imessage will have an option of selecting which device you sends it to otherwise the missus will get bored of having to read all my geeky texts etc