Android :: How To Center Buttons On Screen Horizontally / Vertically?

I've been racking my brain (android newbie here, so not hard to do) for awhile trying to figure out how to accomplish this: Desired Layout using a Relative Layout or something other than AbsoluteLayout which is what this was created with. I'm coming from a Windows programming background where the device adjusts the 'absolute' positioning for you and GUI layout was a non-issue.The first layout works great in the emulator, but doesn't format for my Nexus One or any other screen that differs from the emulator size. I expected this because it's absolutely positioned, but haven't found a solution that will format correctly for different screen sizes. My goal is to have the layout work for different screen sizes and in portrait / landscape. Here's the Code that I'm currently using: [main.xml]
<?xml version="1.0" encoding="utf-8"?>
<AbsoluteLayout
android:layout_width="fill_parent"
android:layout_height="fill_parent"
xmlns:android="http://schemas.android.com/apk/res/android"
><Button
android:id="@+id/Button01"
android:layout_width="188px"
android:layout_height="100px"
android:text="A"
android:layout_y="50px" android:layout_x="65px" android:textSize="48sp"/>