Can an x-president be impeached? (To , at least , strip him of his pension and secret service protection)
Judgment in Cases of Impeachment shall not extend further than to removal from Office, and disqualification to hold and enjoy any Office of honor, Trust or Profit under the United States: but the Party convicted shall nevertheless be liable and subject to Indictment, Trial, Judgment and Punishment, according to Law.
Considering the fact that former presidents are still afforded the use of the title "president" and given lifetime pensions as well as other financial benefits, I would think that based on the way I read this (and granted I'm no Constitutional scholar) that yes a former president could be impeached. It is also interesting to note that the Bent One signed legislation giving all presidents who follow him Secret Service protection for only ten years after leaving office insted of lifetime protection (though this will almost certainly be changed back in light of the War on Terror).
They might be able to if he was convicted of a felony .. but don't hold your breath.