Concept Drifting Datasets in Weka (arff) format:
Spam Assassin Corpus (Gradual Concept Drift)
Mailing Lists (Instant Concept Drift)
You can find the description of both datasets in the following paper:
I. Katakis, G. Tsoumakas, I. Vlahavas, "Dynamic Feature Space and Incremental Feature Selection for the Classification of Textual Data Streams", ECML/PKDD-2006 International Workshop on Knowledge Discovery from Data Streams (IWKDDS-2006), Berlin, Germany.