If the Time Capsule is owned by the AppleCare protection plan holder and is used with a Mac covered by the AppleCare protection plan, it is covered by the computer's plan. Apple does not provide a plan for the Time Capsule as a standalone purchase.
I'm interested in this topic as well. I bought my iMac in May 2008 and TC in June 2008. I was hoping that as my one year is approaching that if I get AppleCare, the 3 year coverage would be for both devices as they are being operated together. Anyone care to comment?