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Lawsuit: T-Mobile must pay for breaking lifetime price guarantee
(arstechnica.com)
This is a most excellent place for technology news and articles.
Marketing promises effectively constitute a binding unilateral offer, for the purposes of contract law. When a customer signs up, you also have acceptance, consideration, and intention, thus forming a valid contract. Carlill v Carbolic Smoke Ball Company is the classic case in English contract law; the principles are basically the same in the US.