At least if you just look at the third party applications that you can use to update your Twitter account. Twitstat released a top 20 most used updating clients list today, based on the accounts tracked by @twitstat. Naturally, this means that the numbers are far from official or final, but it does give a hint on how people update Twitter.
A solid #1 is of course the web interface, clocking in at just over 32%. Perhaps a bit more surprising is the fact that TweetDeck is a solid second at 12%, while the link pumping Twitterfeed is #3 at just below 7%. Twhirl is just under 6%. Check out the complete list here. Hat tip: The Next Web