While the number of Twitter users is still relatively low, the majority of them will be early Tech adopters and likely to have a smartphone of some description, a client with as much flexibility as Twhirl or Tweetdeck (which are used on the desktop computers of many Twitter users) would command a lot of respect, and potentially a high number of sales.
And if you are a Twitter fan, don't forget to follow @AAS, @allaboutngage, @ewanspence, @rafeblandford and @stevelitchfield (although Steve still can't see the point...).
S60 Twitter clients currently under development include Gravity (see Symbian Guru's early preview here) and TwitS60. If you want to Tweet from the home screen your best is probably going to a WRT widget. For example, here is a widget, put together by Rafe, which loads the m.dabr.co.uk site (an alternative interface to Twitter). It's just a shortcut and you can achieve the same thing (on the homescreen) with a bookmark, but the widget has the advantage of having a customised icon and being a stand alone 'application' (separate to the browser).
Edited: Added All About N-Gage's Twitter presence and notes on in development Twitter applications.