If you want to seriously get into the firewood business, first figure out what the rules are for where you can sell to. With all the new pests out there, some states are really clamping down on sales areas (Emerald Ash Borer, Asian Longhorn Beetle, etc.). The agriculture or forestry departments may require licenses and so on.
I'm wondering if making/renting/buying a firewood processor would be worth your while once you get going, or if as Thomas says, work out a deal with someone who has one. Some guys near me are advertising splitting services, I don't know if guys like that would consider working for share or are cash only. If you don't already have one, consider getting a small dump truck.
How about a band mill, like the woodmizer? Depending on what you have for local building inspection requirements, you could have everything you need to make your own barns and sheds with roughsawn siding (board-and-batten or board-on-board, there's even a place near me with live edge clapboards). If you have a planer, you could cut all your own trim boards for the new house. Depending on who's already in business for similar things near you, rough sawn native lumber can be a decent market for the woodworking community (again, check the local regs). If you have guys that "turn" near you, bowl blanks (short logs split in half), too, especially for things that have interesting color or grain, burls and so on. Being willing to work with those sorts of guys to get them what they want won't necessarily get you a lot of cash, but it might help you move a few more trees.