#Python - 44

I'm a Python newbie, so any critique is welcome. Also, not sure if using the *character's* `lower()` is disallowed, since it's not by the rules.  (It says only the string's `lower()` is.)

    print("".join(c.lower() for c in input("")))

If that *is* disallowed, then here's a 49 character version:

    print("".join(chr(ord(c)+32) for c in input("")))