the original code involved a simple hardware switch between pin 7 on the Imp to ground to trigger Twitter response.
It would be worth finding out:
What voltage is on that pin when it is floating.
How low the voltage needs to be pulled down before the Imp will reliably detect that the switch has been operated.
How much current needs to be sunk in order to achieve that.
If the answers come out compatible with an Arduino digital pin then you would be able to connect it directly to the Arduino without needing any external drive components.