How does iMessage decide when to send messages from either my email address or my phone number? Some of my friends receive them from my phone number, and some receive them from my email address. How does this work?
Under "Settings" and then "Messages" on each device (in this case, your iPhone) you should be able to enter your preference. I believe the iPhone defaults to your phone number, but as I am using Messages Beta now instead of iChat, I typically send from my .me account when talking to other friends with iOS or OSX devices.
The reason I do this is for the continuity of conversations across devices.
Check out your settings under "Receive At" under the messages preferences on your iPhone.
I hope my limited knowledge and experience helps a bit.
I know mine is under receive at 2 addresses, email and phone, but I can't change the options between the two, and I just want to know what makes it sometimes send from my email and sometimes from my phone.