IRU stands for Individual Responsible Unit. This is the term AT&T uses to refer to a "company discount" where the customer (or individual) pays for the bill on the account.
If you are a current AT&T account holder with an IRU discount, and you want to purchase an iPhone 3G, go to an AT&T Retail Store, and not an Apple Retail Store.
Apple Retail cannot apply the IRU discount because the IRU discount is actually a list of many companies that offer a discount to their employees. Only AT&T's systems are capable of correctly applying the IRU discount to the contract at point of sale.
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