Google has finally released an update to Gmail that includes calendar integration. Whenever you compose a mail you may 'add event info'. Then when the other person receives the email they have the option of accepting, declining, or setting it as a maybe on their own calendar.
Furthermore, much like how Gmail can auto-detect addresses and place a Map it text on the side, Gmail can now auto-detect events and allow you to place it on your calendar.
One last thing is that Gmail now has the "Google Gmail Calendar" links on the top left of the screen like Calendar has.