So... in a startup... im the techie/developer of the business, my partner is the business side.
While, I'm developing this elaborate application... what should he be doing? What do business people do during this time? Especially during the beginning/early stages of the developer developing?
Can he be taking the ideas/mockups to potential customers to gleen some feedback and make some contacts. On top of that he should be doing some market research and building a potential customer list with contact details so when you are done he or both of you can hit the phones and get in touch straight away rather than having to go find customers at that point. IMO he should be helping you get your job done more quickly and setting up as much as possible to make the next logical stage in your business as plain sailing as it can be, getting 'all your chairs set out' so to speak. Even if you don't have something 'workable' or fancy to show a potential customer you can do mockups with something like Balsamiq Mockups which will enable you to get convey the concept and look at usability issues. It might help you nip out some wrong assumptions you've made about your product before you dig in.
They should be planning a few things.
These 4 things will put you off on the right step at first. Even if the marketing won't start right away, it might help in the development of the product.
Joel Spolsky has said that he feels his role is to move chairs out of the developer's way. I think that this is probably true at many stages. A leader, or a good manager, has to find ways to support those around themselves instead of trying to get their nose into things. Then as the company takes of their role gets more involved with being the face of a company, but they still have to get chairs out of people's way.