RFC: http://area51.phpbb.com/phpBB/viewtopic.php?f=108&t=42403

Move Inbox to top of UCP Private Messages making it the default module when going to that tab.

When people want to go to their private messages (essentially the inbox) from the UCP they get faced with a compose PM page. When you open up your email you don't get a new email screen, you get your inbox. The inbox allows you to compose a new PM or view existing PMs and it makes more sense to have it there as the portal for private messages.