Answer: when John McCain says it is.
Ok, cheap shot, but the depth of dishonesty coming out of the McCain campaign on the Small Business tax thing is overwhelming. Read this CNN fact-check article for an in-depth review, but here are the basic facts:
1) When McCain claims Obama's plan would raise taxes on 50%+ of small businesses, he's using an extremely, shall we say 'unique', definition of small business. By McCain's definition, Obama and McCain themselves actually count as small business owners, because both get royalties from their book sales.
2) The $250K number from Obama's plan refers to income, not revenue. That's an extremely important distinction. My parents own a small business, and while I don't know for sure, I'm pretty sure they take in a good bit more than $250K of revenue a year. But by the time you get done with cost of goods, depreciation, salaries, rent, utilities, supplies, yada yada yada; that more-than-$250K of revenue has turned into less-than-$250K of income. Ergo, Mom and Dad won't see their taxes raised a dime. I can't wait to pound my dad over the head with that when they come down to visit next weekend :P