It seems like there is no voice command in Siri to directly update your status on twitter.
Am I missing something? I thought that with iOS 5 Twitter is tightly integrated into iOS?!
Because Twitter offers to update your status via SMS, there is an easy workaround.
Now you can update your Twitter status via Siri:
Enable SMS for Twitter
In order for the workaround to work, you need to enable the SMS feature for Twitter first.
You can do this either online by editing your profile on twitter.com or send the text message
International Twitter SMS codes
You can get a full list of the world-wide twitter short codes here.
Some countries or providers do not support short codes. You have to use long codes instead. Fees may apply:
You can also go to the Twitter app and press the microphone button on the keyboard.
As Siri is in beta not all features are available, this is something that could be added in the full release.
For completeness, in iOS 6, posting to Twitter is now a fully-supported feature through Siri. Just say "Tweet ..."