If you've received over 200 payments AND at least $20,000 from a single payment processor in 2012, you should receive a 1099-K. In fact, you'll receive a 1099-K from each payment processor (e.g. PayPal, Amazon Payments, Etsy Direct Checkout, Google Checkout, etc.), and only from payment processors, where you meet these requirements.
If you don't meet BOTH of these requirements you should not receive a 1099-K.
For specifics from the IRS, check out the document posted here: http://www.irs.gov/pub/irs-pdf/i1099k.pdf, page 3, right column.
And if you are familiar with the 1099-K and would like a refresher course, please see the 1099-K refresher course.