I haven't had any luck getting an answer from Google on this, and I couldn't find this on their tour, but the new compose and reply screens don't appear to have an area where you can set the from address if you have other hosts set up in your account.This is a problem for me, as I use Gmail for multiple e-mail addresses that all forward to the same place. For now I've set it to go back to the old compose screen, but once I'm forced to use the new one I'm not sure how (or if) it can be done.
Set from address with Gmail's new compose
gmail
Click on the dropdown on the From field, you can switch between sending from email address