It works on Mac OS 10.5 and higher.
The global hotkey for a new Tweet is:
Ctrl Alt Cmd t
Yes there is a twittia: url scheme.
Nice to know
There was a original Twittia 1 which was written in Ruby-Cocoa, it was quite ok back then, but the code was a mess. And then along came Tweetie and I lost the desire to write my own Twitter client, because it was so much further then twittia and looked nicer and so on.
But now Tweetie wasn't update for a really long time, and I really wanted to see retweets in my stream so I decided to rip out the look from Twittia 1 and implement it in a new much slimmer version.