What is a good way to protect the home button on my iphone 4 so it doesn't get dust in it?
I really like Gelaskins (http://gelaskins.com/) for this and have them on my 4 and iPad.  They're nothing but removable decals that help avoid scratching the device (no screen protection), so they don't get in the way of cases, sleeves, pockets, or whatever.  You'll see on their product shots the there is a little gap around the home button.  You don't have to take this off, which actually looks better anyway with an artistic skin.

Plus, you can easily tell which phone is yours without doing something that would lower the resale value, such as engraving.