The String object is not the same as an character array which is expected by the wire library. Seehttp://arduino.cc/en/Reference/StringObject You need to use the toCharArray() to create the char array. Unfortunately the page toCharArray() - Arduino Reference does not describe the params needed.
// toCharArray() copies the internal representation of a string to an array of char.
// buf = pointer to an array of char
// bufzsize = size of the array copied to
void String::toCharArray(char *buf, unsigned int bufsize)