Turn off Edge

Your iPhone automatically chooses the best connection. Edge or 3G. If 3G isn't available then it automatically switches to Edge. If you want to turn off Edge, then it sounds like to me that you want to turn off your phone signal. You can do this by turning Airplane Mode on under Settings. This shuts off the Edge/3G radio.

Other than that, you can't just turn off Edge.

modular747, if you have an unlimited plan currently, AT&T has to honor your contract and keep it until your contract is up.

You can do so only if you jailbreak your iPhone. Apple doesn't expose the feature in the user interface.
Frankly it's very rare I've ever needed to do it. But there have been times and locations that I can not get the phone to stay on 3G despite there being a very strong signal.

I hope OS4 has the option because it's necessary. You don't need neither EDGE or 3G to make a phone call. On top of it, AT&T doesn't know the diffrerence between celular and WI-FI data thus they will overcharge you. They did it to my brother-and-law while he used wi-fi at my house. If the iphone momentarily drops the wi-fi signal it'll switch to 3G or EDGE unless it's passing data simultaneously with wi-fi.

I hope OS4 has the option because it's necessary. You don't need neither EDGE or 3G to make a phone call.

iOS 4 does have an option to turn off all cell network data, but not specifically or only Edge.

On top of it, AT&T doesn't know the diffrerence between celular and WI-FI data thus they will overcharge you. They did it to my brother-and-law while he used wi-fi at my house.

How did they "overcharge" your brother-in-law when the only data plan ATT offered (until 6/7) was unlimited?

This does some necessary. I switched to the 200MB plan from unlimited because I checked like the last 12 months of my usage and I always hovered around 160MB. I figured I could save the $15 since I never used more than 200MB. Then I got a text 10 days into my billing cycle telling me I was over 65% of my data. I didn't use my phone anymore than usual so I'm not sure why its saying I've used so much. Since then, I tried to absolutely avoid any data unless I'm on wifi but when I check my usage on the phone, it'll say I've used like 14MB in like the first 4 hrs of my day. I'm really starting to be suspicious about this. I'm at the point where I'm even afraid to have my phone on because I do not want to be billed &15 just for going over a few MB.

I already called and reported it but they said since my bill has not officially been calculated, there was nothing they could do.

Is anyone else being affected in this way?

Yes, and if you renew your contract (i.e. by upgrading or adding a family plan) you can keep the unlimited data plan in the new contract. It's not clear how long that will continue, and if you switch to one of the limited plans, you can't go back to unlimited.

You can't.

my wife's 3G is stuck on the EDGE network while my 3G has no problem getting 3G network service--and while we're both at home.

why the disparity?

we've done restore, reset