Jason,
http://www.apple.com/support/products/proplan.html
The AppleCare Protection Plan covers your Mac, as well as an AirPort Extreme Card, an AirPort Express or AirPort Extreme Base Station, Time Capsule, MacBook Air SuperDrive, and Apple RAM used with your Mac. Mac mini, Mac Pro, and MacBook Pro customers may also register one Apple display for coverage, provided that the Mac and the display are purchased together.
Note that displays are specifically called out as having to be purchased with the Mac. The Airport, Time Capsule, and others are not.
If you have a Mac under Applecare, the Time Capsule is covered, even if you did not buy it at the same time, otherwise the wording would be as it is for the Apple displays.