I couldn't find anything about this online, but it might be old news. I found out that I can drag and drop calendar items from the Entourage calendar view right into my iCal window. I tried dragging to the icon of iCal, and that didn't seem to work.
The other small problem with this method is that when you do drop an item onto iCal, it becomes an hour long by default -- so you'll have to adjust it accordingly. The nice part is that all the information from the item is brought over.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060329085211554