It's true, they never said on-screen what warp scale they were using on Enterprise
. I suppose it could have been the TOS one. I was basing my comment on statements made by Andre Bormanis around the time of Enterprise
's premiere. He said (I think this was in Star Trek: The Magazine) that they were planning to use the TNG warp scale for the prequel series. Why that is, I'm not certain; I guess it's because it was the one used in the majority of the shows in the franchise -- it was the one fans and writers were most familiar with, and it had clearly defined speeds and limitations.
But I don't know... maybe the scale used in Enterprise
was completely different from the ones in both TOS and TNG. Perhaps the warp scale was reevaluated and redefined every century or so (this might also explain how the E-D goes to Warp 13 in the alternate future of "All Good Things...")
Still, good theories all around. Let the great debate continue!