There's an episode on CBBC of "Lazytown", a kids' show from Iceland of the "eating fruit and getting enough sleep gives me the energy to be a muscular annoying man!" variety. BBC have a woman (uncredited) overlayed like a subtitles track, only she's conveying the speech and sounds with sign language.
The interesting thing is it appears to have been done entirely in one take - I haven't been watching all the time, but had my eye on it for about half the programme and didn't notice any fadeouts/cuts. Signing a whole episode of a somewhat hyperactive children's show in one go, conveying the tone of voice with body language and doing a couple of songs on the way - that is damn impressive!