While I'm more than guilty of committing this crime, I've grown more and more aware of and irritated by interruptions at the office that TOTALLY WRECK THE WORK-FLOW. As Jason Fried of 37 Signals puts it, "your work-day gets broken up into work-moments."
One pop-in disrupts your stream-of-consciousness which can take minutes to get back on track. Multiply this over the course of a week and you find your productive time is more hacked up than the co-ed victims in a B-grade slasher movie.
To help alleviate this, I developed a decision-tree to help folks in my office determine if the pending interruption is truly needed. This is taped to my door. While it is not 100% bullet-proof, the interruptions have dropped dramatically.