I'm planning on buying the iPhone 4S from the Apple store when it comes out next week (waiting in line in NYC shouldn't be too bad). In the past, how many unlocked iPhones will they let you purchase at the Apple store?
Selling unlocked phones is a fairly new thing for Apple retail. They've never been part of a product launch - so looking back, the precedent is NIL. There is great precedent for any new phone launch to have lines and quotas for the first 30 to 60 days after a product introduction. There also is precedent for a huge shipment on launch day - then limited stock for a few weeks as things sell faster than they can be made.
I would simply stop by your store and get to know the business team if you have business needs. Until the policy is announced, it should be handled like any other phone in terms of what payment is allowed (cash or not) as well as purchase limits.
The last launch, it was one phone per person while availability was limited and one web order at a time. Once the stores stopped selling out of inventory on a daily basis, it was easier to just walk in and get a phone. Last time I checked, the store has to bring out a different physical box when I purchased an unlocked phone ( I changed my mind about AT&T contract half way through the process). The current stock does ship as an AT&T phone or an unlocked GSM phone - they are not the same SKU.
I suppose the computers could use a common stock and "unlock" the device when selling, but that would be different than what happens today. I do know that the iOS app to reserve product worked very well last launch, so do think about getting that installed if you are keen to get a few phones as soon as iOS 5 launches.
There is currently no limit to the number of iPhones a person may buy. However, during the launch of a new product Apple has sometimes had to impose a limit. The original iPhone, for example, had a limit of two per person/credit card (they disallowed cash sales so that people couldn't get around it).
In addition to limiting the actual sale, Apple has sometimes limited the number of devices someone can pre-order. For the iPhone 4 last year you could pre-order at most 2 per person/credit card.