Error Readout:
error: A valid signing identity matching this profile could not be found in  your keychain

The Problem:
In Keychain Access it reads the error above.

The Solution:
This can be cause by several issues. The highest probability is, you didn't install the certificate you created in the distribution or development page of "certificates" in the iPhone Provisioning Portal.

Error Readout:
Code Sign error: The identity 'iPhone Distribution' doesn't match any valid certificate/private key pair in the default keychain

The Problem:
I try and build my application and I get the error above

The Solution:
Many things can cause this error. The error is probably caused by one of the steps being done incorrectly in the Program User Guide. Most won't like reading this, but the best way to fix this is to go back and follow, very precisely, the steps in "Program User Guide." In case the link changes you can get to the pdf by going to http://developer.apple.com/iphone, logging in, clicking "iPhone Provisioning Profile" and in the box on the left hand side of the screen titled "Provisioning Resources," you can download the Program User Guide pdf.

Explanation:
Yes, we ALL wish this process could be more streamlined, and require less reading, but it doesn't. If you want to get that app out there, suck it up, read the 60 pages [lots of pictures :) ] and be done with it.