Some baby monitors are using "encryption". I don't know if they are actually using encryption or just privacy codes. Even if they are using encryption, I doubt it's very secure stuff (probably 8 bit or something).
Our baby monitor uses DECT - it's basically a cordless telephone. While I wouldn't expect it to be encrypted to any significant degree, in most cases a baby monitor is either broadcasting silence with a little background room noise, or it's turned off. Once a monitor starts broadcasting noise, a parent intervenes.