It's a radio signal. It's broadcast and if you are in range, you can tune in and listen. There's no possible way this can be construed as "wiretapping".
If you are in line with a telephone microwave tower and have the appropriate radio, you are able to tune in and listening. That does not make it not wiretapping.
It's illegal either way. But to answer your question: GSM and CDMA have weak encryption. GSM at least has optional encryption, and your phone won't tell you if it's being sent in the clear.
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).
Several years ago, I cloned the ESN of my phone to another phone of the same model (Samsung SCH-A680? on Sprint). When I tried turning them both on (next to one another) and calling them, they'd both ring, and answering both would result in hearing the audio in both ears for a short time (presumably until something got out of sync). If there is some token "encryption", it certainly isn't up to modern standards.
