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Archy (George),
You make a good point. There may in fact be other explanations for the red shift of light. Although, the "tired light" hypothesis has been somewhat discredited, I've read recently of a different approach.
String Theory predicts the existence of gravitons (see Wikipedia: Gravitons). Scientific experiments are currently underway in an attempt to detect gravitational waves which "carry" gravitons (analagous to light waves and photons). The experiments cannot detect individual gravitons. However, if gravitons have any mass, then fields of gravitons might cause light to lose energy over large distances i.e cause a red shift.
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