I imagine posting to Twitter will be part of the Planner eventually, but in case anyone is interested, here's how to get the Imp to post to Twitter using http://open.sen.se
Create an account on http://open.sen.se
and then create a new channel – choose "Arduino & other"
On the custom device screen name your device and choose "other" for "device is"
The other settings should be fine (HTTP Post etc).
You'll then add a feed. Give it a name and choose "input - data flows from device to sen.se"
In the drop down for "what type of data" choose "text".
Save the feed and click finish.
In the device settings choose "anything else" from the side menu and you'll see instructions of how to POST to sen.se
Make a note of the feed id and then copy your API key.
Now go to this gist, copy the php code and save it as "opensense.php"- we need this to handle the post from the Planner and to then construct json to send to sen.se
Open the opensense.php file in the text editor of your choice and change the feed id and api key settings to the ones you noted down before. Upload it to your server and remember the url - we'll use this in the plan in a moment.
Now in the Planner create a new piece of code - and paste in the Squirrel code from the gist above. It's a very basic example that has a button on pin 1 and GND.
In the Planner add a HTTP Request vimp and drag a noodle from the Imp node to this HTTP Request vimp. You should be able to just press ok. Click the settings icon in the vimp and paste in the url of the location of the php file on your server that you created before.
Finally, back in open.sen.se find the device you created earlier and add a new app. Choose the twitter app, authenticate it and in the template just simply type %VALUE.
Now when you press the button it should tweet.
Hope that's useful!