The law authorizing the rebate checks (H.R. 5140) specifically limits qualifying income from Social Security benefits to "any amount received by the taxpayer by reason of entitlement to - (A) a monthly benefit under Title II of the Social Security Act, or (B) a tier 1 railroad retirement benefit."
Supplemental Security Income is funded by general tax revenues, not Social Security taxes. It is designed to help aged, blind, and disabled people, who have little or no income by providing cash to meet basic needs for food, clothing, and shelter.
Specific Questions? Call the IRS
Since everybody's tax situation is different, and final qualification for the rebate checks will be decided by the IRS based on each individual's 2007 tax return, all specific questions regarding the rebate checks should be directed to the IRS. You can call the IRS at: Toll-Free, 1-800-829-1040 Hours of Operation: Monday – Friday, 7:00 a.m. – 10:00 p.m. your local time (Alaska & Hawaii follow Pacific Time).
More Tax Rebate Resources
Tax Rebate Information Directory
IRS Publishes 2008 Tax Rebate Check Details
IRS Launches Tax Rebate Information Center
Rebate-only Tax Filers May Now File for Free
Will I Have to Pay Taxes On My Rebate?
To Get $300 Rebate, Kids Must Have SS Number
SSI and the Tax Rebate Checks
What if I Filed Before the Tax Rebate was Approved?
Tax Rebate Checks Become Scammer's "Bait"