I was a long time user of TravelTracker on the Palm and it was an excellent application, but the best thing about it was that it integrated well with the Palm Calendar (and hence to any other calendar synced to the Palm).

My understanding is that Apple has not provided APIs for app vendors to manipulate the calendar so that isn't present in the iPhone version. If I had not cut way back on my travel over the last couple years I would be buying this in a flash.