If you buy the Nexus One with the T-Mobile plan, $79.99/month includes 500 minutes and unlimited data. It's detailed on the first step of the order process: https://www.google.com/phone/choose
Isn't the "T-Mobile Even More individual 500 Plan" $39.99 + $30 for unlimited data? This would come to $69.99, not $79.99.
Unlimited Domestic Messaging is an additional $10.00 not included in Android Unlimited Web.
Data is same as AT&T $30/month. I just traded in my HTC MDA for switching to AT&T and the iPhone 3GS. It cost about $5 more per month at AT&T total.
My same plan at T-Mobile was $100, new one at AT&T is $105ish.
I thought the tmo data plan was $25 per month... do you have a link to the $30 pricing?