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.