And here's some more generic feedback after trying out your app: Buttons should be hildon.Button instances (with FINGER size, probably) The gtk.Entry you have should be hildon.Entry (for the styling)
Also, it's not really obvious that the entry box belongs to the "Search" button (in addition to other things). A nicer design would be to open a dialog with a text entry box when the "Search" button is clicked and open the search results in another view (StackableWindow).
When I'm not entering any authentication data, Clicking on "timeline"+"refresh" should probably use a Hildon Note instead of a dialog with the title (and ideally with a 401 error, you could show the login dialog, as this is where most users would want to go in this situation).
The "Exit" button in the appmenu is redundant, as there is the "X" button in the titlebar already. What does "Invert" in the app menu do?
The context menu should pop up at the location of the mouse coordinates, and should have the widget name set to "hildon-context-sensitive-menu" to get the correct styling.
The "TwitPic!" item should use a hildon file chooser instead of the GTK+ one.
I would also suggest either opening a new subview for different content (e.g. "Trends", etc..) or at least changing the window title so that the user knows which "mode" he/she is currently in (e.g. "Witter - Search for maebar" or "Witter - Your timeline").
Oh, and a feature request: Add support for identi.ca Thanks for the great application - I especially like the "search" functionality, as the only other twitter app that I know of does not yet have this functionality