I'm sure everyone knows this by now, but I am submitting it because I've not see anything about it on the web.
Last night, I received an email with an .ics file attached, and Mail identified it as an iCal file, so I opened it and all of a sudden, there's a new event in iCal. So I asked the sender if he was using iCal and he said no, it was Outlook. Apparently Mail takes Outlook meeting requests and converts them to iCal events.
[robg adds: I think this is relatively new behavior. I tested from my work Outlook account to my home email, and sure enough, double-clicking the .ics attachment in Mail opened iCal. iCal popped up an Accept/Decline dialog box, which included the requester's name, meeting name and time, and asking which calendar I'd like to add the event to. I declined the event, and was surprised to see that I was then offered the ability to send the decline notice to the originator. Very cool...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040128191309718