Tripit makes simple something that could be very complicated. It takes all the various emailed itineraries you have for a trip -- flights, hotel bookings, car rentals and so on -- and turns them into one master itinerary with your car rental booking following your flight and preceding your arrival at your hotel. You can view your itinerary online; the iPhone app gives you offline access to it, and with one click you can add the whole thing to your iCal or Entourage calendar.
Now Tripit has added the ability to edit your itinerary in that iPhone app; for example, you can change a flight time in case of cancellation or delay. You can also swipe-delete items that are no longer needed, and Tripit will notify you of conflicts if you make changes to flights.
The price is still right too; the app is free with ads, or there's a $1.99 ad-free version and then the Pro anual supscription version at US$49 a year.
(Edited to clarify ad-free pricing)