I don't have a problem attaching rat muscles to a machine. There's no sanctity of life issues there.
However, the promise of nanotechnology is often centered on medical applications. The idea of rat muscles crawling around inside my body to remove a cancer tumour is not very appealing. On the other hand, if I had cancer, and there was little alternative, bring on those rat muscles!!!
Rat cells? I'm waiting for the PETA rebuttal before I conclude there are no moral issues here. :)