This should be clear to anyone with a grasp on personal finances. For those that struggle in this area (a large % of the population), the recent housing debacle should be a good real-life example.
Although I don’t think any generic multiple of one’s income is an accurate indicator of the price of house he or she can afford.
My friend moved from Boston when he got a job with NASA just outside San Francisco. He bought a used Aerostar van, and found some girl on craigslist renting out an extra room for $300/month. It’s certainly not glamorous, but he never has trouble with meeting his financial obligations, even when unforseen things go wrong…which is inevitable.