I keep hitting this issue. ...but think about it, you are investing all of this time, and all of these resources in your business, do you have an agreement about equity? Have you accounted for everything you have spent together?
Incorporating is not only important insofar as it shields you from liability, may provide certain tax advantages, and allows your business to effectively function as a third person who can enter into contracts and the like - it is also an opportunity for you and your cofounder to really sit down and talk about what the business is, and where it's going.
Any attorney you talk to (and I think you should talk to an attorney) will have pointed, specific questions for you about your business that may help focus you on what you are doing, and make clear everyone's rights.
Incorporating later, in addition to putting off the benefits of a corporate entity, can, if you do not have a rock solid founder agreement in place, potentially endanger your enterprise, as disagreements about who worked when, and what that might be worth can easily arise.
I am not your lawyer - this is information, not legal advice.