Could be a few things.....
Increase mileage recently? Too much too fast could be the reason....
How old are yer kicks? They are either worn out, a cheaper type or they are the wrong shoe for your foot. I'd venture worn out if you have done a lot of running.
I used to run a LOT so I feel/felt your pain. Running through the pain isn't the answer, it's only going to make it worse.
Check this site for a lot of good running info....
http://www.runnersworld.com/topic/0,...85-0-0,00.html