I personally believe it should be anything sub ambient. I have benched with my water loop and had below 0c temps when it was around -15c in my area. This isn't something that effects me at all since I'm in the OC league anyway, but it does create an unfair advantage in the enthusiast league.
On a side not though, we could argue all day long about what is fair and what isn't when the fact is we can never create a 100% fair environment for the enthusiast league since someone from Yakutsk in Russia could easily get sub zero temperatures with a decent air cooler at certain times of the year. But without proof who is to say that he's sub zero? Also would he be sub zero throughout the entire benching session so should he be in the OC league or Enthusiast league?
For example, I was benching a 4870 on DICE but for my last run on 3DMark 03, the temps were 1c at the start of the bench and 17c by the end. The pot still had DICE in it but throughout the entire run it never went below 0c because I was in a rush to get that done before I moved on to something else. Where would I be placed then since I used DICE but I never went below 0c?