jbenny: "OK, I got my N800 booting OS2008 from either flash or a 1GB SD card. Now, this may be a stupid question, but how do I now install OS2007 without overwriting everything I just did? I would prefer OS2007 on the flash and OS2008 on the card."

I believe you can just flash the OS without losing anything that isn't in the flash memory. That is, you will still have your card with OS2008 and a (damn small, if the the whole card is 1GB) partition for data on the card. I think you'll still have the boot manager installed; if not, at most you should have to do just steps 4, 5, 6, and 8 (which isn't that bad).

But I think you won't even have to do that. Report back and let us know!