Yes, you can have the $20 (may be different for your company and service provider) corporate plan with an iPhone. In order to have this plan, you are not allowed to be in contract. If you want to purchase the iPhone, you must do so at the full, undiscounted price(or purchase from a 3rd party) and pay any ETF for ending your contract.
They don't care if you're using a iPhone, jPhone, or a toaster... As long as they don't pay for it.
I think you can only buy the phone(or toaster) from an actual store.
Edit: so typing that, I realized there's two corporate plans. FAN discounted and the employee plan.
If your company has a deal to apply a FAN discount to your AT&T plan, that does not require a contract. You may purchase the iPhone at full price and shove a micro sim in it at your conveinience or buy a subsidized phone and enter a contract. Your choice. Just like any other person. FAN discounted get no special treatment other than a discount on the main line's voice plan(that's right no discount on the texting, data, or 2nd+ line if you have a family plan). I you leave the company, you keep your discount.
If you sell AT&T phones at any reseller, you're offered the employee plan. Your AT&T rep has to be the one in control of your account and has to be the one to create your account. That's the plan I posted about originally. If you leave the company, you lose your plan.
I opted for the FAN discount and a contract. Those phones ain't cheap!