You heard it from me before, and I will say it again, (send me kudos when it is announced) Leopard will amazingly allow you to share your computer to your iPhone and Apple TV. You will be able to check email and surf on your TV in your living room, and you will be able to operate your computer from your phone. You will also be able to share all media content seemlessly between the three. Screen sharing on Apple TV and iPhone allows your computer to house the memory and power and simply uses the your television and iphone to mirror the screen. It's simply genius. Everyone will want all three and a .Mac account to boot. Also, it will be unveiled in June. This is why it's Top Secret.
Since iPhone doesn't support 3G - sharing iPhone to computer is going to be SLOOOOW.
iPhone supports WIFI... but if I'm in range of my home WIFI, why wouldn't I just use my Mac in the first place?
There are plenty of solutions available for sharing existing phones to computers, as some one pointed out, there is microsoft remote client for pocketpc. There are VNC clients for other phones, such as Symbian.
If OSX 10.5 does allow sharing, you can bet it'll be based upon a opensource product like VNC - just like the Apple Remote desktop is.