Is there enough demand for your product or service? This is something you'll want to investigate in more detail as you develop your business plan. At this point though, what's important is to do some preliminary research. Searches on Google, Hoovers or Bizminer will help you study a particular industry, and you can often drill down your research to a particular state or city. Your search at Google is of course, free, but you'll often find for a small investment at sites such as Hoovers or Bizminer, you'll get meaningful data for your market vertical, which you can start analyzing right away. It's also not a bad idea to survey buyers on their purchase behaviors and perceptions towards your product or service. Arranging a questionnaire or focus group can give you some useful insight into how potential buyers react to your product or service. If it's reasonable, consider giving away product or service trials and then follow_up to evaluate user expectations and experiences. If you don't have demand for your product or service, it really doesn't matter how great it is anyways, right?
I have written business plans for all manner of industries: a coin operated jukebox company, airlines, travel companies, new product launches, and anti_aging product companies. It is not necessary to have a passion for the product or the company to write or develop a business plan. What you must have is a passion for aggregating information, getting involved with and understanding the service or product, and understanding the financials of the product or service. By financials I am not referring to having a CPA before you undertake the task, but rather understanding the presentation of the information and analysis/ numbers to support the activity being planned. Financials are important because they are the score card in the world of commerce.