In very rough numbers (good enough for government work) for 2009:

US GDP: $15 trillion

Federal Government spends 20% = $3 trillion

Breakdown of the $3 trillion

• 20% Defense
• 20% Medicare/Medicaid
• 20% Social Security
• 10% Interest on Debt
• 30% Everything else

The full budget (and historical trends) is available online. Despite the din, not much has changed. The inexorable rise of medicare / social security continues, though. Medicare and social security represented 20% of the federal budget in 1971. Today they represent 40% and growing…