So, taking Eclipse RCP, munching it with twitter4j, adding a bit of Apache Derby and other stuff, I ended up with the following prototype in a couple of hours:
It has a virtual table, so it loads tweets on-demand from the local database when you scroll down. It only reloads the latest 20 tweets from Twitter.com in online mode. There is an offline-mode to decrease the amount of network bandwidth and service utilization. Twitter is already pretty slow. The client caches all the profile images for faster loading. It displays the time of the tweet and highlights URLs. I had a feature in it which resolves shortened URLs and displays the real ones on hover, but i disabled it as I've got problems displaying a nice hover. Hover's in Eclipse RCP/SWT/JFace just don't look very nice, compared to all the new Web 2.0 or Mac designs found in modern apps today.
What other features could be put into yet-another-twitter-client? Tag clouds? URL clouds? Charts? Google Maps? Automatic translations like Babelfish?