A little bit more pythonic version

Il me semble...

def rot13_v2_1(c):
 C = c.upper()
 d = ord(c)-ord(C)
 if 'A' <= C <= 'Z':
 return chr(((ord(C) - ord('A') + 13) % 26 + ord('A')) + d)
 elif '0'<=C <='9':
 return chr(((ord(C) - ord('0') + 5) % 10 + ord('0')))
 else:
 return c
def rot13_v2(chaine):
 return "".join( map(rot13_v2_1, chaine) )
if __name__=="__main__":
 print "ROT13 DECODER/ENCODER"
 print
 text = raw_input("Type your text : ")
 print "The result : " + rot13_v2(text)