I found an interview with Thomas Haden Church (‘Jack’) who explains it this way:

Q: What’s the title of this movie mean to you?

A: The title of the movie is open to interpretation. We talked about it often. You go through a door sideways. Or the little roads that go off to the wineries in the Santa Ynez valley are sideways of the 101. But it’s also these little portals that open up, and the experience. These guys are at such a crossroads in their lives. Their lives have been largely defined by failure and you would think the prospect of marriage, which is supposed to be bountiful and hopeful, it’s just really another kind of tangential thing in his life. I mean, he’s so kind of desultory about it even at the beginning. Whenever Miles comes, he can’t wait to get away from it. One week away from being married and he can’t wait to get away from his new in-laws and his fiancée. And there’s actually some stuff that we shot that was cut out of the movie, these very antagonistic phone calls with his fiancée that Alexander [Payne] took out of the movie.

Source: http://romanticmovies.about.com/od/sideways/a/sideth101204.htm

I never understood the name of the movie. Did I miss something along the line or was there something in the script that didn’t make it to the screen? I figured there’s be some speech about how you need to keep the cork moist and be able to see the precipitate.