When I was looking at this awhile ago, these guys had gotten pretty far with the concept. I think they had a blog article detailing why POS companies were a pain to work with and in some cases very expensive (they charge partner companies for access to their "APIs")

A business plan might be a good idea. Who is going to pay for the service, are you going to charge the bars or the consumer or both?

OpenTable is also doing this now for restaurants they have the added advantage of having a dedicated PC with their software in each restaurant already.