$1 isn't exactly a micro-transaction though, is it? Maybe by comparison to buying a full album...but I think the article is about more 'temporal' type exchanges (reading a post ad-free) whereas the iTunes "purchase"/license does indicate a tangible item, so to speak.

In my opinion iTunes is an outlier due to several factors: 1. The tie-in with the hardware (Apple iPod), 2. High-profile discussion about piracy and "punishment" avenues that were being discussed prominently, and 3. Steve Jobs working in the background to capitalize on 1 & 2 for very favorable terms with record labels enabling the price point (which was again maneuvered by Tim Cook with iTunes Music getting good deals for the new service).