As @paaj says, any AirPlay enabled speaker can be a current network endpoint. Whether a particular AirPlay device also implements RAAT and becomes RoonReady will depend on whether the device can be upgraded through software or firmware and the manufacturer writing and releasing it. AirPlay functionality won’t be lost when RAAT is released, but AirPlay doesn’t do hi res like RAAT.
@JChrisG, there’s a pretty good chance that Squeezebox support will be in the next major release.