Stackoverflow reputation - worth mentioning in personal statement?
If you find value in participating in an activity, then you should feel free to put it in your personal statement. It's no better or worse, say, than including your participation in hosting a neighbourhood PHP class, just that your "neighbourhood" is on the web. Just note the following:


Assessors may not understand what Stackoverflow is, so you may need to spend valuable space informing them about it. You need to describe not only what you do, but why you do it. 
You may have many other activities in which you find value that you would like to feature. This means that the space you use to describe Stackoverflow will compete with space that you could use to describe other things. At its heart, isn't this an economic decision?
Understand that some assessors may look you up on Stackoverflow and assess your work differently. These sites are public, after all. Are you prepared for this to happen?


Good luck!