For all the reasons Garbz stated, I really dislike Live View. And that said, it does have its uses. However, using phase detection AF is possible. It's a custom function that needs to be enabled (usually off by default) manually. On the 450D, it's C.FnIII (8), Auto Focus/Drive AF during Live View shooting. Setting it to 1: Quick mode, when you depress the focus button, the camera will lock down the mirror and focus using the AF sensor, and flip it back up when it's done. (2: Live mode uses contrast detection, as it keeps the screen on, and sucks battery power like nothing else.) If you want to change AF points, you have to get out of Live View on the 450D, as the AF point selection button turns into a zoom instead in Live View, but thankfully, the selected AF point is shown in Live View by a small white box, so it's not like you're focusing blind.