All answers suggest that in such cases IRS's definition of "constructively received" should be used.... which states (per IRS Publication 538) "Income is constructively received when an amount is credited to your account or made available to you without restriction. You do not need to have possession of it. If you authorize someone to be your agent and receive income for you, you are considered to have received it when your agent receives it. Income is not constructively received if your control of its receipt is subject to substantial restrictions or limitations."
In the case of the end-of-year mailing of payments that didn't arrive in your hand until the following year, despite the date on the check, you didn't constructively receive the money until the following year. So it's taxable income, in most cases, in the year you got the check.
Keep in mind, the person who sent you the 1099 is probably going to provide the 1099 to the IRS. But if you have good records of the date you received the money, you will have no problems.
