Ensign won't resign but he deserves to be called a hypocrite
As a matter of fact, I will say that I support his decision not to resign. That's because it is up to the voters of Nevada to decide whether they feel that whatever he may have done in his personal life disqualifies him from serving. I'm not endorsing his affair, but let's be honest here-- if all the adulterers in Congress were caught and resigned, it would be sort of like baseball without steroid users; the number of faces that'd be missing would likely be massive. However, I've never felt that what people do in their personal life has, or should have, any affect on how well they do their job. I disagree with almost everything that Senator Ensign stands for but that's a completely separate issue from his personal life.
However, where he does deserve a box on the ears for this is in the personal hypocrisy department. In fact, (as Nate Silver points out) Ensign has in the past called for Bill Clinton and Larry Craig to resign after they got in trouble for their sexual exploits.
Not that Ensign is the first Republican to engage in this kind of hypocrisy of course. Remember that the impeachment charge against Bill Clinton was led in the House by Speaker Newt Gingrich and pit-bull hatchet man Dan Burton. As we now know, both Gingrich and Burton were covering up their own affairs at the same time they were moralizing about Bill Clinton's. And let's not forget Republicans like Larry Craig and David Vitter who got where they were precisely by preaching about morality.
Yes, Democrat Eliot Spitzer became Governor of New York the same way, but he got what he had coming-- Spitzer could have ridden out getting caught doing a one-night stand with a prostitute but the hypocrisy of a guy who had run on his record as a prosecutor of sending other people to prison for the same thing pretty much guaranteed that he had no other option than to resign.
Ensign hasn't sent anyone to prison but he has certainly preached about morality and for that he deserves to be branded as a hypocrite.
Say what you will about Bill Clinton, but at least he never criticized anyone else's sex life.