Apple introduced AppleCare+ alongside the release of the iPhone 4S, and there's been plenty of confusion surrounding the new warranty plan since then. Instead of the typical AppleCare plan that has been in place for years, AppleCare+ offers extended coverage to new iPhone customers with new requirements.
If you bought an iPhone 4S on launch day, you're unable to buy AppleCare+ now. If you plan on purchasing the iPhone 4S, or didn't buy it on launch day, you have a 30-day window to purchase the plan before you're left out in the unprotected cold.