What about BASE58 encoding the URL? Like for example flickr does.
# note the missing lowercase L and the zero etc.
BASE58 = '123456789abcdefghijkmnopqrstuvwxyzABCDEFGHJKLMNPQRSTUVWXYZ'
url = ''
while node_id >= 58:
div, mod = divmod(node_id, 58)
url = BASE58[mod] + url
node_id = int(div)
return 'http://short.com/%s' % BASE58[node_id] + url
Turning that back into a number isn't a big deal either.