October 2013 Update: “Unlimited” has been clarified to mean 3G/4G data speeds for 2.5 GB per month, throttled to 2G data speed after that. Both AT&T and T-Mobile SIM cards are now available, with the newer ones allowing 4G / LTE speeds if you bring your own 4G HSPA+ / LTE-compatible phone. Price dropped to $6.99 with free shipping. See screenshot below.
You can also now buy the iPhone 4 ($200 refurb) and iPhone 5 ($400 refurb) directly from StraightTalk now at reasonable prices. iPhones sold by Straight Talk are CDMA and work on Verizon networks and their wider coverage. Note that the CDMA iPhone 5 comes factory GSM unlocked as well, so you can even switch to AT&T with the SIM cards above. Remember these come with no contract and also go with the $45 unlimited plan. Details below.