Get Rich Slowly has an interesting post about what to do if Uncle Sam comes calling and you're too broke to pay.

"The first route is via Form 9465, Installment Agreement Request (PDF). You supply the terms based on your ability to pay over the next few years. If the amount owed is under $10,000, the IRS will accept most reasonable offers to pay within 36 months. Otherwise, larger amounts may require further correspondence and data gathering for the IRS."

Taxpayer Panic: What To Do If You Can't Pay Uncle Sam [Get Rich Slowly]