Nokia 770 doesn't have (right now... or I haven't found it) a user-friendly client for twitter and for some unknown reason you can't post through the twitter web page. After googling I found a website that includes a list of twitter clients. mitter seems to be in the future the default twitter client because their plans include to port to Maemo and use Hildon, but not now; so if you are planning to post from your device then you have to figure it out.
Finally, after trying I found a solution by using the power of vim and its plugins, of course you will need to install vim and curl packages in your device to allow twittering from it.
By the way, don't forget that the BACK button (the button below the arrows left the screen) in your device acts as ESC key.
Update 2008-03-07: If you add the twitter user to your gtalk you will be able to twitter by sending him messages, nice isn't it?