After launching in 2007 and getting a dedicated app in early 2012, iTunes U has seen one billion content downloads. In a release, Apple said that the online education tool supports over 2,500 public courses and an unspecified "thousands" of smaller private ones, with over 1,200 universities and colleges offering courses through it. While the company touts massive enrollment in classes from American universities like Ohio State (which it says enrolled 100,000 students its first year), most iTunes U downloads come from other countries: over 60 percent of the billion downloads (which are of course material, not the app itself) come from outside the US.
Update: AllThingsD has also received word from Apple that the company has sold 4.5 million iPads into US educational institutions over the product's lifespan, giving us some hard adoption numbers. TechCrunch says Apple has sold "far more than 8 million" iPads to educational institutions worldwide.