We talked to about 20 investing entities (maybe 5 or 6 VC firms, a mess of angels, and a few angel groups). Exactly 1 asked for a business plan and they were the lowest quality entity with the least success/experience.
Fred Wilson (one of the top consumer VCs in the world) said that 17 out of 26 of his "successful" investments (with exits) utterly changed their business model between investment and exit. In other words, a business plan has a roughly 60% chance of being obsolete.
Planning is good. You should rough out some numbers. Build a spreadsheet or two. If it's ad supported, get a sense of CPMs in your market and try to figure out how many ad views you'd need to support a real business. If it's a B2B offering, rough out what you can charge, investigate low cost distribution angles (SEO, SEM, etc). In short, prove to yourself (and others) that your business has some legs to stand on.
Write this down in an executive summary and be able to talk about it in some more detail... But writing a formal business plan? Chances are you don't know what business you're really in yet. And if you think you do, there are pretty good odds that you're wrong.
Of course, if you have a mature startup (a year or two old, a pile of customers), then some more structured planning might be appropriate.