If there is a date and/or time in a message in Mail, you can simply hover your mouse over that day and/or time to produce a pull-down menu that lets you create an iCal event with that date and time. It seems to smartly title the event as well. You just simply hit the create event after any tweaking you may want to do, and voila, you have a new calendar entry.
Also, if a message contains a street address, phone number, FAX number, etc., you are presented with the same type of pull down menu allowing you to add that information to your addressbook. You can add it as a new contact or associate it with an existing contact.
[robg adds: This capability comes from the new 'data detectors' in 10.5, and they seem to work quite well, at least in my initial testing. At the moment, it seems they only work in Mail (I tested iChat and TextEdit); hopefully that will change over time to include other Apple and non-Apple programs.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20071025160504230