[sl956]

That's a way to protect the trackball if you lay the phone face down on a flat surface. On the G1, it is not that pocket unfriendly.

[Andys]

On my G1 it also protects the menu button (which also unlocks the phone from locked mode) from being pressed accidentally.

While you can configure an unlock touch gesture, I haven't bothered yet because I don't find that the menu button is being pressed accidentally in my pocket.