June 3, 2011 
First off, whoever framed this idea of 10,000 steps per day was a genius. It’s a much simpler metric to tell people. Since I run (on my good days) a few miles, I had to translate that to miles to understand what I needed to do. 10,000 steps is 5 miles. If companies were going around telling everyone they had to walk (or run) 5 miles a day, they would lose people quickly. [One likely question here is whether running and walking burn the same calories.]
It seems like everyone is pushing 10,000 steps per day right now – Here’s Kaiser’s Program.
Of course, on the flipside, this can be confusing since you have this program. You have the government’s recommendation of 30 minutes of exercise a day (which isn’t 5 miles for most people). You have advertisements for supplements. You have workout commercials. You have diet information. What works?
(In searching for 10,000 steps information, I found this Dr. Oz video which I’ll share on weight loss tips.)
As an interesting side note, I was wearing my pedometer the other day talking with a physician. He asked what it was. When I told him it was a pedometer, he asked why I wore it. I talked to him about measuring my steps to get to 10,000 a day. He’d never heard of the concept.
Enabling Healthy Decisions 
In my opinion, framing in terms of time is easier — most everyone has access to inexpensive yet accurate clocks of some sort, and trust them to be accurate. Most people also have some internal concept of time, so if a clock were inaccurate, most people could fairly quickly identify the inaccuracy.
I don’t think the same can be said of measuring steps. Many people do not have pedometers, and while there are many cheap options available, I question their accuracy. That skepticism comes, in part, from having little internal concept of steps with which to gut check the pedometer’s data. It could be right or wrong and I’ve have no way to know, which makes it harder to trust.