In case you hadn't noticed, you can't directly buy AppleCare on any Apple AirPort product such as the Airport Express, Airport Extreme and Apple Time Capsule. However, any AppleCare coverage on your Mac extends to these devices automatically.
Currently, if you look at the "Apple Hardware Coverage" for any "Applecare Protection Plan" that covers a Macintosh computer, you'll see that Apple includes AirPort in that coverage:
AirPort must be purchased up to two years before your Mac purchase or during the term of your AppleCare Protection Plan coverage.
Even if the one-year warranty has expired on your AirPort, if you have AppleCare on your Mac, you may still have protection. In my experience, not all Apple support people know this policy and often you have to get an "exception" to the standard 1 year warranty. Occasionally I've had to call Apple Customer Relations to get a "CS code" to approve the repair.
This somewhat hidden benefit of AppleCare can save you potentially hundreds of dollars if the hard drive on your Time Capsule dies in year 2 or 3. (As ever, though, bear in mind that the existence of AppleCare doesn't mean you don't have basic consumer rights which you can ask to exercise even if you haven't paid up.)
AppleCare Protection Plan [Apple]