Here's how to move any single event from iCal to Entourage. After creating the event in iCal, simply drag the event to the Entourage dock icon, and a window then pops up in Entourage, showing the saved event information in your default calendar. Unfortunately, Entourage does not transfer the US/Eastern time zone correctly, but the event times will be correct in Entourage if the time zone in iCal is set to "floating."
ICS files can also be dropped onto the dock icon, although success for multiple events may not be complete.
[robg adds: I haven't tested this one...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20051114112210259