gistfile1.py

#!/usr/bin/env python

import httplib

def check(letter):
    conn = httplib.HTTPConnection("twitter.com")
    conn.request("HEAD", "/naoya_"+letter)
    return conn.getresponse().status == 200

if __name__ == '__main__':
    import string
    print [x for x in string.lowercase if check(x)]

# result (2011-01-30):
# ['a', 'c', 'd', 'f', 'h', 'i', 'k', 'm', 'n', 'o', 'r', 's', 't', 'u', 'y']