It's actually not (edit: it actually is illegal either way for cell signals in particular, see below), according to federal code. We'll have to dig up some old cordless phone interception cases, but if you weren't trying to defraud the intercepted line (like racking up a bill), you're more or less in the clear.
California and other states had stricter laws about telephone conversations in particular, but as others have noted: when laws are particular about their domain, that almost always means they are not applicable by analogy elsewhere.
Edit: ah, someone in the Ars comments brought up a good point: cell phone frequencies are specially regulated and auctioned by the FCC, and intercepting them is indeed prohibited (by my reading at least). Since your connection to your cordless phone base station is itself not a common carrier, though, it is considered publicly accessible (by federal code), and so it is not illegal to intercept it (when it's not encrypted).
California and other states had stricter laws about telephone conversations in particular, but as others have noted: when laws are particular about their domain, that almost always means they are not applicable by analogy elsewhere.
Edit: ah, someone in the Ars comments brought up a good point: cell phone frequencies are specially regulated and auctioned by the FCC, and intercepting them is indeed prohibited (by my reading at least). Since your connection to your cordless phone base station is itself not a common carrier, though, it is considered publicly accessible (by federal code), and so it is not illegal to intercept it (when it's not encrypted).