According to news reports from Germany, it was the "right kind of ash" after all. It appears that the Eyjafjallajökull ash consists of basalt, which only melts at 1,200°C, a temperature in the engines normally only reached during take-off. The ash of many other volcanoes (including Pinatubo, Mount St. Helens or volcanoes in the Andes) consists of andesite, which melts at below 1,000°C, a temperature in the engines reached much more frequently.
The abrasive properties of basalt appear to be the same as those of andesite though.
Ergebnisse des Messflugs: Vulkanasche schwebt in 3,5 bis 6 Kilometer Höhe - SPIEGEL ONLINE - Nachrichten - Wissenschaft