UPDATE: This script uses basic auth and no longer works as of Aug 31, 2010. Please use the updated version with OAuth support

This is a really simple mercurial 1.2 hook for posting incoming changesets to twitter. It requires python-twitter.
File: hgtwitter.py

'''
[extensions]
hgext.hgtwitter=

[hooks]
incoming.notify = python:hgext.hgtwitter.hook

[twitter]
username = twitter_username
password = twitter_password
'''
from mercurial import cmdutil, templater
import twitter

tweet_template = '''
|{root|basename}:{rev}|
{desc|strip}
'''.strip()

def hook(ui, repo, hooktype, node=None, source=None, **kwargs):
    ctx = repo[node]
    tuser = ui.config('twitter', 'username')
    tpass = ui.config('twitter', 'password')

    t = cmdutil.changeset_templater(ui=ui, repo=repo,
                                   patch=False, diffopts=None,
                                   mapfile=None, buffered=False)
    t.use_template(templater.parsestring(tweet_template,
                                        quoted=False))

    ui.pushbuffer()
    t.show(ctx, changes=ctx.changeset(), root=repo.root)
    tweet = ui.popbuffer()

    if len(tweet) > 140:
        tweet = tweet[:139] + u"\u2026"

    api = twitter.Api(username=tuser, password=tpass)
    status = api.PostUpdate(tweet)

t = cmdutil.changeset_templater(ui=ui, repo=repo,
                                   patch=False, mapfile=None,
                                   buffered=False)