Looks like the Pope resigned so quickly to make sure he wasn't still Pope when the warrant for his arrest became public.
"Pope Benedict, Joseph Ratzinger, has scheduled a meeting with Italian President Giorgio Napolitano for Saturday, February 23 to discuss securing protection and immunity from prosecution from the Italian government, according to Italian media sources.
Ratzinger's meeting follows upon the apparent receipt by the Vatican of a diplomatic note from an undisclosed European government on February 4, stating its intention to issue an arrest warrant for Ratzinger, who resigned from his pontificate less than a week later.
In response to the February 23 meeting, the International Tribunal into Crimes of Church and State (ITCCS), through its field Secretary, Rev. Kevin Annett, has written to President Napolitano, asking him to refrain from assisting Ratzinger in evading justice."