Apple hasn't (yet?) added posting to Twitter to the tricks in Siri's bag, but if all you want to do is tweet out a quick status, you can add it yourself.
It's simple: Navigate to Twitter from Mobile Safari, jump into the mobile settings panel and make sure you have your mobile phone number setup with the service. Twitter will then spit out a number for you to text message Twitter updates to. For confirmation's sake, send a quick status update to the number to make sure the text is pushed to your Twitter timeline.
For more, see Clay Russel's write up below.