A tax return filed in April of this year is for 2005. You'd be filing a tax return in April 2007 for income made during January 1, 2006 to December 31, 2006 (which is the accounting period used by most businesses).

This applies to both questions. If I write you a check for $500 for various services, you'll be claiming it as income next year, whether you file 1040 Schedule C (S-P) or 1120 (corporation).