Is there any point to a separate window for making a new tweet?
Just that it can be invoked without having to open the main window (ie, via contractor).
I would dock it like a modal if it's invoked from the twitter app. Otherwise, you could integrate it into the notification system. I have said for a while that apps like notify-osd need interactivity.
It might not always make sense for it to be modal, especially if you need to see something else on Twitter as a reference such as an @ name or # tag. Twitter for Mac does do an interesting thing where it's a popover if you're replying though.