I have to agree with James on this. There may be many ways to answer this but when researching comps I personally treat short sales as I do every other possible comp--It depends on what the purpose of my research is. If I'm doing a very narrow, specialized look at comps for a specific property, I would not include it unless my subject is also a short sale. For a wider spectrum market analysis, it really depends on what other comps you have to work with. If you have enough non-short sale comps, you don't need to use it.
But, as mentioned below, a single short sale does not constitute a true representation of a particular market area (except for instances when it does, lol).
Another way to look at this is let's say you're planning to sell your house. Your neighbors have the same house plan you do with similar upgrades and view. They're getting a divorce and are selling their house for $100,000, though they paid $300,000 and it's worth approximately $500,000. I would not consider this a comp because the price is artificially low and should not have any bearing on the market value of surrounding properties. Now let's pretend this is a short sale. Same answer.
If a majority of houses in this area are being sold for $100,000 by people getting divorces, then you will most likely not be able to sell yours for $500,000. This definately does affect your market value. But one house probably won't.