You can't "just" unlock it. Unlocking is risky: it voids your warranty, makes the process of updating your OS difficult or impossible, and has a very real chance of bricking your phone.
Unlocking does not void your warrantee. I've had many unlocked iPhones replaced under warrantee. That said, if you brick it while unlocking (and the 4S tools are very new), you won't be covered.
Perhaps your local Apple Store doesn't check for unlocks before warranty replacement, but unauthorized unlocking absolutely voids your warranty in principle and Apple would be well within their rights to refuse service. If you don't believe me here's the relevant section of the warranty (written by Apple in bold):
"This warranty does not apply: [...] to an Apple Product that has been modified to alter functionality or capability without the written permission of Apple".
(For what it's worth, while software unlocking is usually not a huge or dangerous affair, there is not currently a software unlock for any iPhone beyond the iPhone 3G.)
Even though the 4S is supposedly a GSM/CDMA combined phone, its still locked to individual carriers, unless you happened to buy the unlocked version at nearly $800 USD.
I chose Sprint as my smartphone carrier after seeing a TV ad talking about how they don't throttle their data at all.
I have the unlimited data plan for my mobile device, and am quite happy with it.
I also purchased from Sprint a netbook with an embedded WiMax (4G cellular) internet connection, and unfortunately, there's no unlimited data plan available for it. I have to settle for a 12GB/mo. plan for about the same price as my Blackberry's plan. It's now the first month I've had my netbook, and after bringing all my offsite files in locally, I'm stuck using public WiFi for the rest of the month since I already used up my 12GB, + $250 for an additional 4GB.
The only problem I've had with Sprint so far is that the netbook's WiFi didn't work out of the box, and I couldn't use it to do my big offsite backup's download. I eventually got the corrupted apps reinstalled & afterward contacted Sprint support to try to get a credit on my overage charge, but they would only credit a third of it citing that I should have waited until it was resolved before finishing my big download project.
Go to verizon ask them if they have a switch bonus..google for the right term to use before talking to the CSR person..that might get you anew iPhone and keeping your same number as part of the free gifts on coming over to Verizon..
