Apple followed up with me late this afternoon to my question about the number of iTunes accounts that had been compromised. Turns out, not that many. Apple told me that an extremely small percentage of users, about 400 of the 150 million iTunes users - that is less than 0.0003% of iTunes users, were impacted.
To the question of whether the iTunes servers themselves were ever in any danger of hackery Apple says that the iTunes servers were not compromised in any way.
Of course, this all comes on the heels of that developer, Thuat Nguyen, who has since been removed from the App Store for violation of the developer terms. His Vietnamese book apps were skyrocketing in the iTunes charts while people's credit cards were getting billed.
Apple says that starting today they're implementing a new security measure to minimize this type of fraud in the future. Basically you'll have to enter your credit card's CCV code a little more often from now on.