TripIt's server generates an iCal feed which you can subscribe to using the iPhone's calendar application. It is separate from MobileMe, Google or whatever calendar server you prefer, though you can usually set up your web calendar to automatically display TripIt events alongside your own calendar events.
The best feature, in my opinion, is that the TripIt calendar is time zone independent, meaning that the calendar feed will always show up in terms of the time zone that your phone/calendar app is currently using. This is really useful for planning transpacific trips (where you often arrive before you leave) and complex trips between several time zones. I generally plan my schedule at each destination using the TripIt web app, where everything always shows up in the local time wherever it is happening. It will then be automatically reflected on my iPhone and Google calendars in Japan time, and I can use those times to tell my wife/friends/co-workers when I am arriving, when to call or not call me, etc. If I need to refer to the itinerary in local time before my trip, I open it in the TripIt web app or use their iPhone app. Once I get to the destination, I set the iPhone to local time, and my itinerary shows up in local time on the iPhone's calendar.