Just click and drag an event as if you were moving it to a new time. As soon as the cursor leaves the main calendar view area it will turn into a small "summary" version of the event you are moving. Just drop this on the desktop, and the .ics file will be created automatically.
If your desktop is not in view, you can drag the event to the top or bottom iCal window border to create the droppable event, and then invoke Exposé's Show Desktop mode (F11, by default) to drop it on the desktop.

