If you own an iPhone 4 with Verizon, you're not eligible for an upgrade--at least not at the subsidized prices of $199 for the basic 16 GB model. Verizon has been selling the iPhone only since February, which means that its Apple-loving customers haven't finished one year on their two-year contract. If you use Verizon's service for another smartphone but want to jump over to the iPhone 4S, you can check your upgrade eligibility at .

Since Sprint is new to the iPhone game, there are no existing iPhone customers with that carrier. But if you own another phone through Sprint, you can check your upgrade eligibility through the . Sprint requires that it be 20 months since your last contract started before you're eligible for the upgrade.

(While Sprint won't take preorders for the iPhone 4S until Friday, bargain hunters can get their hands on a $99 iPhone through the carrier right now-- on Thursday.)

AT&T charges a one-time fee of $18 to upgrade your phone; Verizon doesn't charge a fee to upgrade phones after a contract has been completed, but does charge a one-time $35 activation fee for new service.