If it is in 1080i, the set will lock into 16:9 with no ability to switch to 4:3.
If it is in 480p, you will still have the option in the menu to switch between 4:3 and 16:9.
But there are no 1080i games yet.
I have the KP-53XBR300, which is similar. When the TV receives a signal (or the signal type changes) it will be displayed on screen. This is only valid for Input 5, if you have the XBOX connected to any other inputs you will only get 480i.
Usually, there is an option on the TV which tells you what kind of a signal the current input is showing. On my Mits. 55807, it's called simply 'Info'. When I select it (on the remote), I get either 480i, 480p, or 1080i (Mist. doesn't support 720p).
I can easily tell if I'm watching 480P on my LT150 Projector as I switch back and forth with the press of a button. But when I get a game and pop it in on my Toshiba 40x81 RPTV I find it a little more difficult.