Why can't I boot into recovery mode? I think I'm actually missing something here still. Everything I read suggests I need to boot into recovery mode as a first step, and that I ought to be able to do that by holding down the "home" key while I'm starting up. Instead I get a bang (!) in a triangle icon with a phone next to it.

Have I skipped a step or is there something wrong with my phone? I know there's something wrong with my phone ... that's why I embarked on this project to begin with. But could that be some how preventing me from booting to recovery mode? Or is the problem a skipped step?
Edit: Apparently download and recovery modes are merged for the G1.  Just press Alt-L on your physical keyboard while in Download mode, and Recovery will show up.

What you're seeing is Download Mode.  The key combination is usually subtly different.  In the case of my phone, it's just a matter of having the phone connected to USB or not.  You should try it with and without USB, as well as removing the battery and doing a fresh boot.  (You can also remove the battery, plug into USB, and reinsert the battery and try).  Also try other keys -- you already know using Back instead gets you to that device info screen.