Title pretty much says it all.
I'm going to an iPhone 5 next month and want to sell my iPhone 4S by then.
I bought it in December or January of 2011 and purchased AppleCare+ with it. AppleCare+ has never been used once and will allow you two have two incidents of accidental damage (prior to 2 years from the date I bought it) and they will replace your phone for $49/per incident.
This is all I need to do to transfer the warranty/service plan: http://support.apple.com/kb/HE58
It's the white model, 64GB, unlocked by AT&T once I bought it (I had no commitment on my contract). This will work with any GSM carrier that supports the handset frequencies. I've been using it on StraightTalk (which uses AT&T's 3G network) and paying 48 dollars per month for unlimited talk, text, and data. I can set the phone up so you can use it out of the box with a straight talk sim card if you'd like (otherwise you have to configure data).
Message me for more details!