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Customers who already paid $379 can expect to receive a $100 refund from Google. While T-Mobile hasn't officially discussed what makes a customer upgrade-eligible, we know that customers who still have a year left in their contracts have been offered the $279 price.
Here's the current Nexus One pricing system:
$529 - Unlocked.
$279 - Eligible T-Mobile Upgrade Customers
$179 - New T-Mobile Customer who signs a 2-year contract