Botanicalls is a open project that takes time working.
You can buy the kit and assemble or otherwise you have all information on the web to build and why not make improvements?
The project where arduino is a twitter notification as better success at marketing today is BakerTweet. This is a funny device where the baker select that product just out of the oven and this is published in twitter.
What to does this serve? Those who do not live in town many times when we go to buy bread bakers are selling day-old bread. By this we know when we can go for fresh bread, cakes, buns, croissants, ... ;)
Another open project is the Tweet a Watt, this project is very interesting and useful because we can monitor the consumption of a device by the Twitter. From my point of view is an expensive project, but with a little imagination we can reduce cost if we are forced.
Oficial Tiwitter library
Silly projects (my point of view ;) ):
Bubblino
KickBee