The home button is held in by the glass digitizer - there are tabs on the left and right sides of the button.
This picture has a pretty good view:
http://s1.guide-images.ifixit.com/igi/RIIpDuoiMhXR3YK4.thumbnail
So, if you need to replace the home button itself, you are going to have to remove the digitizer, and will need these to adhere it back in place:
http://www.ifixit.com/iPhone-Parts/iPhone-3G-Front-Panel-Adhesive-Strips/IF137-006