I know that general approach to interaction design in Bitcoin assumes minimal to no difference between payer and payee, and generally I agree with this approach. 

However, for the sake of my PoS development this assumption is wrong by default, as PoS is a specialized hardware, and one who cared to buy and install it is probably not in the same situation as the other party that didn't care to by anything dedicated. 

In short - from PoS point of view there is a customer and a merchant. And my goal is to make thing work in assumption of fast and reliable connection on merchant side and no connection requirement at all from customer side. 

I didn't put a silly example, as of my experience there are really a lot of places where cellphone connection isn't good enough for reliable Bitcoin operation. However, if we're talking about merchant establishments - we can hope for private local WiFi or wired connection on PoS side, so PoS internet connection shouldn't be an issue. So this is the use case I'm designing around and this is why bluetooth based BIP70 implementation is important for me.

I partly agree with Mike on user interface and IOU idea, but I have no intention to implement anything like that right now.