The main figure is the second line from the bottom. Over two years (the length of a service contract), the unlimited voice and data plan plus the cost of the subsidized phone is $3,799 for an iPhone on AT&T, but only $2,579 for a Nexus One on T-Mobile. That means the Nexus One will cost you $610 less a year. You will want to buy an 8 or 16GB memory card for your Nexus, so let's round it down to $600, shall we?
Also check out the Droid on Verizon. Ouch. Let's hope that when the Nexus One comes to that carrier it's competitive with T-Mobile.
Of course, money's not everything (or if it is, check out the Palm Pre). T-Mobile is not the country's best carrier, and the iPhone has a better integrated music store and way more apps. But the Nexus One plan forces you to ask what you're getting for the extra money the iPhone costs.
The useful BillShrink site has other money-saving ideas, too.