How to update your Twitter status from Siri on the iPhone 4S
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.
Source: iPhone Addict