Another way to add To Do items to iCal.
One of the the goals of the Getting Things Done methodology is to get tasks out of your head and into a place where you can track them. That means making extensive use of organized lists.
Although iCal isn’t the best tool for GTD, it certainly is workable. (I have yet to find a better solution, although I admit I haven’t been looking very hard.) The iCal GTD Widget makes it quick and easy to add a To Do item to iCal without launching iCal.

Install the iCal GTD widget and place a copy of it on your Dashboard. Then, when an idea flashes into your mind, press F12 to display the Dashboard (and the widget) and enter a reminder for your idea into the widget’s edit box. You can choose a calendar from the pop-up menu if desired. Press return and the note is added to iCal as a To Do list item for the calendar you chose.
Can’t be much simpler than that.