Tweeting from iOS will just get better and easier when iOS 6 gets released in fall. All you would is pull down the notification bar and then press 'Tap to tweet'. But while you wait, you can get that twitter widget right now on your notification centre using a jailbreak tweak on your device running iOS 5.
Go to Cydia and search for iOs 6 Twitter Widget and install it.
After installation, restart springboard and then go to Settings >> Notifications and enable the twitter widget. Now you simply pull down the notification bar and tweet from there.
That's it. You should also check out how to get iOS 6 photos menu in iOs 5.
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