Why doesn't iMessage send from my phone number?

I'm unable to use iMessage on my iPhone4 with my cellphone number. It will only work by using my email addresses that are on my phone. "Waiting for activation" shows on the Messages Settings page, and only shows emails that can work with my iMessage. Why won't it allow me to use my cell number? I've tried reading forums to correct the issue, but none of the reccommendations work. Help!