SS is reduced (they call it "an adjustment") based on your filing
status and your AGI. This year for example: Married filing jointly
with AGI of $164,000.01 to $204,000.00 will have an adjustment of
$25.80.
The adjustment is based on the previous year's tax return and it is
added to your deduction for Medicare Part B premium.
A higher AGI means a larger reduction of the SS check. The max for
2008 would be an AGI of more than $410,000.00 for married filing
jointly and would result in a monthly reduction of $142.00.
As others have pointed out, the accumulation of W2Gs can impact the AGI
and that impacts the monthly check in the next calendar year.