If you have an event in Outlook's calendar, you can drag this event onto iCal's window, and it will be duplicated in iCal. You can also drag the event onto the iCal icon in the Dock, and you'll get a dialog asking which calendar you want to use, if you have more than one, and it will be added to that calendar.
[kirkmc adds: Works as described. The hint submitted was about dragging onto the window, but when I tried dragging onto the Dock icon, I saw that it worked too. I use BusyCal, and this works when dragging onto the program's Dock icon, but not when dragging onto the calendar window. If you use other calendar programs, it might be nice to try dragging and dropping between them to see what works, and post your results in the comments. Also, I've only tested with with 10.7.]
Mac OS X Hints