I think these are the basic situations that support a COY candidate:
1. Low expectations; exceeded expectations to the point of being a strong playoff contender. Examples: Smith, Harbaugh.
2. Medium expectations; exceeded expectations to the point that the team is one of the few top Super Bowl contenders. Examples: Fisher, Mangini.
3. High expectations (i.e., Super Bowl contender); met expectations. Examples: Coughlin, Tomlin.
Preseason, Belichick would have had high expectations, but the Brady injury immediately dropped expectations... so he falls somewhere between 2 and 3 IMO.
Zorn doesn't qualify IMO, because his team is not a top Super Bowl contender and did not start the season with low expectations.
I don't really buy Sparano... certainly, the difference between 7-4 (Falcons and Ravens) and 6-5 (Dolphins) is enough that he is at best third in that category right now, and in truth, I can't say I see the Dolphins as a strong playoff contender. Plus, the Parcells factor probably takes away a bit from his case.
I think situationally, 1 > 2 > 3 in terms of most potential for the award. In some years, perhaps each category does not have a strong candidate. This year, there are strong candidates in each category, and I seriously doubt Coughlin or Tomlin has a real shot, since they fall in the bottom category... unless the other candidates finish poorly. Probably not Belichick, either, despite losing Brady, for similar reasons. There are a lot of mitigating factors to apply, though, like injuries, schedule difficulty, rookie coach, etc.
I know a lot of this is subjective, but that's how I see it. Based on this kind of thinking, I think Smith is the guy right now, and I think the only other strong contenders right now are Harbaugh, Mangini, and Fisher (in no particular order).
ETA: I don't really see the Bucs as a top Super Bowl contender, and they did not start with low expectations, so I don't see Gruden as being a contender here either.