If you have a fully functional proto, then focus on getting traction. Then see if you really need a business plan or not. If you have traction, investors come knocking on your door.
Investors look for:
#1 Extremely strong management team.
#2 Traction. This is usually tied for 1st place but, as Paul Graham (YCombinator) says, if the team is very smart and capable, then the ideas are less important.
The b-plan is not for investors - they don't read them. The b-plan is strictly for you. It's a great exercise in seeing if you have a sustainable business model. Every entrepreneur should write at least one in their lifetime. On the flip side, I've found that if I'm busy futzing with explaining why my product is great, I'm not WORKING on my product. Plus, it can be really demotivating if you find that the logistics are too difficult, or you start to worry about money because the plan always has to be written ultra conservatively. Unrealistic statements of future revenues impresses no one.
If you still want to write a plan, let me know. I have written award-winning business plans and won a few competitions based on presentations from my plans. If you need pointers let me know, I'd be happy to help. But my 2 cents is to just screw the plan, and work like hell to get traction for your product. Then it'll sell itself.