[apgwoz]

,[.,] is /bin/cat in brainfuck. Though, brainfuck fails miserably at most golfing challenges.

[icebraining]

,[.,] doesn't actually concatenate files, though.

Personally, I think that using golf handicapped languages is the only way to keep it interesting. If you can choose the language, it gets too easy. One could even implement 'cat' as a single character, provided his language of choice.