Surprisingly, there has not yet been a dedicated (non-Java) Twitter client for the S60 platform even though S60 leads the world in market share. There was a very tightly controlled private beta of Gravity going on for a month or so, but I didn't get a chance to try it myself. This morning Gravity went public and I immediately downloaded and installed it on my Nokia 5800 XpressMusic device. After about an hour of usage I can say this application gives me the BEST Twitter experience on a mobile device. I have tried several iPhone and Windows Mobile applications, but find the speed, animations, functionality, multiple client support, and more to be so much better on the 5800.
Multiple client support is important to me because I have a main account and one for Nokia Experts so I can focus on Nokia and S60 related topics there and interact with my followers that may not care to hear about my politics, religion, or personal affairs. I haven't been updating the Nokia Experts Twitter account much because of the lack of multi-client apps and now should be Twittering more with both accounts available in a single application.