1% originally, then a station can go to 4 or 6% without approval if I remember correctly. 10% does need FCC approval and proof of non-interference.
What I would like to see is a completely digital test of HD Radio and how that impacts adjacent and second adjacent interference plots. I mean, if we're going to make things more efficient, lets go all the way and do it right. Besides, it would be interesting to see the parallels between IBOC transmissions and DTV with respect to distance and power used.