Well, right now it's not as big of an issue, since Congress passed a law in 2008 that waived Federal income tax on forgiven debt in distressed sales.  That law applies to sales through 2012.

In the normal case, it's my understanding that it is the cancellation of the debt, not the deficiency, that triggers a tax obligation.  When debt is actually forgiven, the bank notifies the IRS thru a form specifically for that purpose.  That's how the IRS would typically learn that you owe tax on the cancelled debt.   If the lender is reserving the right to collect on the debt later, they are not forgiving the debt and so no form goes to the IRS, and I would imagine it is unlikely that the IRS would come after the taxpayer for the deficiency (although I'm sure that some debts go away without notice to the IRS but the debtor still echnically owes tax).

So, if this is true, nobody should be paying tax on a deficiency and then have someone try to collect on the debt later.

Please do not construe the above as tax advice, etc etc.!