Tethering an iPad from iPhone

Does anyone know if it is possible to tether an iPhone 4 to a iPad WiFi only model ?
That would be great if you could.
I dont think sim cards or data plans can be shared by providers like Rogers.
Ill need to confirm this but Im pretty sure theyre not.
My point with all of this, is that you shouldnt have to buy separate data plans for devices you own that have internet access capability. There should be a way to have these devices share a single internet connection.
The most ideal I can think of is allowing data plans to be shared amongst devices by the provider, this however may take awhile .... in the mean time a tethered connection via bluetooth would work nice. This should work both ways (iPad --> iPhone OR iPhone --> iPad).
Another idea is a simple wireless access point app may also work if allowed to take over the attennas.
Let me know what you think.
Thanks
Ron
Mac Mini and iPhone 4 Mac OS X (10.6.5) and iOS 4