Square's mobile payment system, you know that a critical part of the whole setup is the tiny credit card reader that plugs into your phone's headphone jack -- and in all likelihood, you don't have it. The company just sent out a message to those who've signed up for their service today, saying that a prior hardware shortage is resolved but that it's only half of the problem; the other half, and arguably a more serious one, is that the company needs to find a way to "handle the huge demand for readers and still manage the risk of chargebacks and fraud." So far, those risks have been mitigated by setting limits very low, but customers have apparently complained that they're too low, which is requiring a "rethinking and expanding" of the company's underwriting. Everyone's being assured that they'll be good to go once this whole process is over and clients go through a credit check -- but in the meantime, Square readers might be a little scarce, so it's back to IOUs and pocket change for a while. See the full letter after the break.