Cash flow positive - normal situation where the cash inflows during a period are higher than the cash outflows during the same period. Positive cash flow does not necessarily means profit, and is usually due to a careful management of cash inflows and expenditure.
So, while bring "cash-flow positive" means that the company's cash balance isn't decreasing, it doesn't mean that it's profitable.
I think they have to IPO, and they would have already if it were good news. Not that every tech company needs to IPO, but Dropbox customers have no switching costs and little feature differentiation, so at some point low prices or great support is going to win the market -- implying scale to me. To achieve competitive scale (of operations, not userbase), you need financing, and Dropbox isn't going to issue AAA rated bonds or anything, so it's a matter of time.