Jim Miller says:

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Does DES (or name your favorite encryption algorithm) produce as output all possible cyphertexts of length L, given all possible conbinations of keys and plaintexts of length L?

As Perry points out, for any key, cycling through the 2**64 inputs will produce all 2**64 outputs. On the other hand, there are (2**64)! mappings of the set of inputs to the set of outputs, and DES only uses 2**56 of those mappings, so there are probably values of P and C for which there is no key K such that C = DES(P,K).