Do you use the Google URL builder to create custom URLs that GA can track? We found that using that significantly increased the Twitter referrals being reported in GA -- from a few hundreds a day to a few thousands.
The Snip-n-Tag plugin in Firefox easily creates Bitly shortened URLs with the GA utm codes added. This makes the creation of URLs that GA can track a breeze.
I add something like &source=twitter onto the URL before I shorten it. Then even if the landing page gets reengineered and put through adifferent shortener, I can see the traffic that came as a result of the twitter links.
This can then be tracked as a campaign or series of campaigns in Yahoo Web Analytics. I expect GA can do similar?