This was definitely too easy. Problem 56 once again demonstrates how simple it is to play with bignums and strings.

(use '[clojure.contrib.math :only (expt)])

(defn digit-sum [n] (reduce + (map #(Character/getNumericValue %) (str n))))

(defn euler-56 []
  (reduce max (for [a (range 100)
                    b (range 100)]
                (digit-sum (expt a b)))))

(time (euler-56)) ;; "Elapsed time: 5568.29599 msecs"

We could probably speed this up if we studied the math more (regarding what and how many digits would be produced by exponentiation), but I like the brevity and simplicity of this solution.

Ciao!