Low (single digit) percentage?
Given that the vast majority of smartphone users (of any particular OS) are going to be pretty much ignorant of data allowances, and how much data they actually use, I would say that if a low, single digit, percentage of WP7 users are actually *reporting* the problem, it is likely far more widespread than MS suggests.
Just because (a) someone doesn't know they have this issue, or (b) doesn't report to MS that they have this issue, doesn't mean that they aren't affected.
I'm not a WP7 user, but I do read sites such as these. If I were, and I'd noticed the excessive data usage, but knew that it was a known issue that MS were looking into, I wouldn't have reported it myself either!