From the number of posts... I guess your car is new?
Idle was really harsh when the car was new and especially cold... mine even stalled two times during a rough idel. :shrug:
After putting some miles it became more smooth and no more rough idle... :clap:
up to 5000 miles mine was a bit rough.
at about 5001 miles the engine switched and was like a different engine.
this happened both on the elise and my corolla XRS.
(same engine, different brainbox)
I had the same issue on my car (2005), and it was due to the idle set screw being set too low so that the ECU was having to control the idle speed. This can cause the idle speed hunting behaviour and low initial idle after letting off the throttle (which can sometimes go low enough to stall the engine). Just properly set the idle with the set screw, and there is a good chance your problem will go away. It did for me, and I had this happen twice - once with the original throttle body, and once again with the new throttle body that was replaced under warranty.