iOS 5 added Twitter integration across various apps. But surprisingly you can't sent a tweet using Siri. Maybe a future update will cure that, but for now there's a workaround. Here's a step by step guide on how you can tweet using Siri on the iPhone 4S.
1. Log in to Twitter. Go to Settings, then select the Mobile tab. Add and then confirm your phone number to your Twitter account.
2. Add a new contact on your iPhone 4S. Choose "Twitter" as the name of your new contact and 404-04
as the phone number. This is Twitter’s SMS shortcode.
3. Using Siri, say the following: "Tell Twitter Sending my first tweet using Siri." Behind the scenes, Siri will technically send a text message to 40404 with your message. What that ultimately means is that you will update your Twitter status using just Siri.