I came here to post pretty much the same thing. When I bought my Nexus One, it was pretty much the top Android phone you could buy, so keeping it for 2 years was not out of the question. Judging by how well the G1 has held its value over the past 2 years (or longer, im not certain) I imagine the sale of my Nexus when I'm ready to upgrade will take a bit of the cost of whatever phone I do upgrade to later down the line.
Combine that with what I save on my tmobile plan and it works out really well for me. Of course I also travel a lot, so an unlocked quad-band phone is a must. Oh, and it's also just a fantastic phone, so it worked out great for me all around.