Twobile is a brand-new app, still in beta, designed to be a Twitter client for Windows Mobile devices. The app is small - only 200K - so keeping it always running shouldn't hog up too much memory. The Twobile app offers many different features, each in a separate tab. You can switch between your friends timeline, your messages, your replies, etc. For active twitters, this may mean a lot of switching needs to be done, but no worse than any other app. However, the layout for viewing tweets does make good use of the screen's real estate by displaying one-line views of the tweets. You can download the software for free from here.