U.S. Cellular will pay you $300 to switch carriers
The conditions of the deal/s may have changed since the initial publishing of this post.
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U.S. Cellular has the Samsung Galaxy S III for just $149.99 on contract
If all of this appeals to you, if you do bring over two lines and activate a smartphone, the carrier will send you a prepaid MasterCard debit card charged up to $300. It's a nice piece of change for doing something that you might have been planning to do anyway.
source: U.SCellular via AndroidCentral
Things that are NOT allowed: