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the 8GB version of the Apple iPhone 4 on AT&T is being offered for $69.99 with a 2-year agreement. If you have an old Apple iPhone 3GS you can trade it in for $100 - $200 based on your capacity of either 16GB or 32GB.
If a tablet is what you are looking for, buying an Apple iPad 2 will award you a $50 Radio Shack gift card, which can be used for purchasing anything else. Probably even invested towards an iPhone 4.
The promotion is in efect as of today and will go on until December 24th. Unless you are waiting for the very last minute to get your Christmas gifts, you may want to get your hands on this deal right now.