Even though April 15 has passed, disabled veterans and others who normally do not file a tax return can still submit a 2007 form to receive a payment. See news release IR-2008-65. Others who have already filed a return may get their payments directly deposited into their accounts starting April 28. News release IR-2008-66 has details.
Check the Economic Stimulus Payments Information Center on IRS.gov for updates. Also in Spanish.