You may not know the term, but you do know how it works, I'm guessing. HIPPO is the high level manager who comes to your project at the last moment and offers an opinion on what to include to make the project a success. And you must consider it, even if the idea is out of scope, past deadline or as crazy as buying BP stock.
I was once leading development of a Web site and had to choose among several color palettes. We tested them with users, and with internal stakeholders before making our final decision -- green. But it was not final, actually. A HIPPO waded in after the decision was made preferring red, and that was that. Another example. While I was at the conference I talked to a project manager who was embarking on an iPhone app simply because the boss thought it would be cool. There were a lot of those stories at the conference, actually.
How to deal with a HIPPO who clearly outweighs you on the org chart? Jim Sterne, a web metrics expert who introduced the HIPPO term at the conference, offered a very good solution.
While you may want to say, "Your opinion, while interesting, is irrelevant," what you need to say is, "Great idea Boss! We'll test it."
Bathe your beta site in red and see if users buy more product. Inventory your customers or look at user data to see how many actually use an iPhone. One of the great advantages of working on the Web is that testing options is relatively easy, and data for analysis easy to come by.
If you can tell your boss that the green site results in a 35% increase in sales over the red version, that's a pretty good argument.
Have HIPPOS stomped around on your work? What's your strategy for keeping them at bay?