I've seen a number of mentions that Mitt Romney is the first losing presidential nominee not to attend the inauguration in 24 years. But that's not really fair. By my count, he's the first losing nominee since Michael Dukakis (24 years ago) who didn't have a current elected position in DC when he was running. Bush (92), Dole (96), Gore (00), Kerry (04), McCain (08). In other words, the other guys basically had to be there, as presidents, vice presidents or senior members of the Senate.
Late Update: Charlie Savage notes that Dole had actually resigned from the Senate in the middle of the 96 campaign. So that slightly breaks the model. But it's Dole who'd been there forever and lived at the Watergate. And constitution may actually say he's still in the Senate despite resigning. In any case, same basic logic applies I think.