It's progressive. You pay 15% on first million you make. 30% on amount thereafter.
EDIT: It looks like I am wrong, ignore me.
EDIT: It looks like I am wrong, ignore me.
The relevant sentence seems to be "If a participating developer surpasses the $1 million threshold, the standard commission rate will apply for the remainder of the year." (emphasis mine)
Apple takes the comission as you go, so it sounds like they take 15% of each sale until you hit $1M, and then 30% for every sale after that.
The next year they'd take 30% right from the start though, so a good year followed by a bad one would be unfortunate.
If you get kicked out, you're stuck paying 30% for at least a year.