Joe Rossignol has details over on MacRumors:
After paying upfront for an AppleCare+ plan for a Mac, the initial coverage period remains three years, but customers now have the option to purchase additional coverage that automatically renews annually until canceled. The new coverage must be purchased within 30 days of the end date of the original coverage, according to Apple.
Previously, there was no way to extend AppleCare+ coverage for a Mac once the initial three-year coverage window elapsed.
I think this is great news. Macs are better-built and longer-lasting than ever, and three years is just not realistic when it comes to how long most people use their computers.
Apple has more information in this support document.