The reading I've done on impeachment indicates an executive may be impeached for a variety of misconduct - and that the timeframe is not limited to time spent in office. While the primary goal of impeachment is as a check on abuse of political power, any conduct that brings dishonor on the office or interferes with a President's ability to carry out his duties is grounds for impeachment.
In my estimation, revelations of wrongdoing before the person held office could merit impeachment. Certainly severe criminal offenses would interfere with the work the President was elected to do, regardless of when the offenses were committed.
Of course, nothing I've seen would indicate this should even be a topic of discussion in regard to VP Cheney. Makes for interesting constitutional chat, though.
You are correct. Thanks for your input, we might not always agree, but it is always welcome.