They may not do so right away, but if this feature isn't available within a month or two after the Verizon iPhone launches I'll be shocked.
First and foremost, AT&T is going to try and do all it can to prevent iPhone users from jumping to Verizon which means they will do everything they can to be as price and feature competitive with Verizon as possible.
Second, they already allow tethering and have a lot of users who are already using this feature. It's not hard on either a Mac or Windows PC to share your internet connection with other devices. There are no doubt already users out there who are tethering and then using their laptops or desktops to share the connection with other devices.
Third, there's a $20 dollar app you can buy called MyWi for jailbroken iPhones that allows you to do this already and has for some time without giving an additional cent to AT&T over and above a standard data plan without having to pay extra for the tethering option if you're smart about how you use it. If they offer this service there is a certain percentage of those users who will go legit. I had a jailbroken iPhone and used MyWi until AT&T offered the tethering option. I didn't like the thought of having a jailbroken phone and having to hold off no new updates, etc, but I needed that feature. When they made it available I restored the phone to it's normal state and switched the the 2 GB plan that offered tethering and went legit. I'm sure there are a certain percentage of users out there who have jailbroken phones that use the app or an app like it who would go legit if AT&T gave them the option.
That sort of leads into my last point which is that AT&T has moved away from the unlimited plan model and moved into a usage based pricing model. Why not allow users to use their iPhone as a mobile hotspot? To do so they'll have to move away from an unlimited plan if they were grandfathered into one and into a tiered usage based pricing plan so they'll be paying for the data they use. Why would AT&T care which device is consuming the data? They shouldn't. All they care about is selling your the service so they can put more money into their pockets. IMO offering the feature is a great way to get more users who were grandfathered into the unlimited iPhone data plan to move away from it. AT&T will get paid for how much data you're using no matter which device it may be.
I realize their goal was probably initially to get you to pay for your iPhone's data connection and then pay seperatly for either a 3G card for you laptop or a mobile hotspot device to power your laptop and other devices but that's just one giant PTA for consumers in terms of the number of devices they'd have to carry around for mobile Internet access so most users wouldn't go down that road even if it was the only road available to them and since their competition is already going down the phone as a mobile hotspot device route AT&T has no choice but to follow.
The bottom line is that this is a feature they'll be offering sooner rather than later and if not then there are likely to be more users jumping to Verizon than would have otherwise been the case.
Right now I'm not planning to move to Verizon. I've been happy with AT&T's service up to this point but being able to use the iPhone as a mobile hotspot without having to jailbreak the device is the kind of feature that would make me jump to Verizon when my contract is up in 6 months. AT&T better either already be offering that feature by then or have a firm date announced that isn't too far in the distant feature for when it will be available or I'll move over to Verizon and won't think twice about it.
I've used them in the past and, for me, their service worked as well as AT&T's has. I happened to already be an AT&T customer when the iPhone came out but it's not like I would hesitate to make a switch. I just don't see the point since AT&T's service has been as good as I think you can expect a cell phone service to be provided AT&T is competitive with it's pricing and features.