Calendar integration is pretty rudimentary at this time, I really need to do better for Exchange.
Right now the app downloads iCal / .ics invites as attachments, letting the user open them. But there is a catch -- the built-in Calendar app from Google can't open these files.
There are "bridge" type apps in Market that respond to AquaMail opening .ics invites, and put them into the device's Calendar.
And yes, I do understand the need to do better.