AT&T stores have begun to offer fully subsidized pricing on the iPhone 3G to select early upgrade customers. According to Apple Insider, the company is waiving the upgrade clause for customers who are approximately 14 months into their 24-month contract and want to purchase an iPhone 3G. Previously, users who were not yet eligible for upgrade pricing were forced to pay $399 for the 8GB model and $499 for the 16GB. The move comes closely after…