This is the reason my wife wanted an iPhone. We have been using Reunion on our Macs for over 10 years, and we are thrilled to now be able to carry our entire genealogy database around in our iPhones! In fact I have three databases on mine: our large family (over 28,000 people), a smaller one of other families, and one of all the U.S. Presidents.
Right here on my phone I can look up any data I want, view pictures of ancestors, and enter new data or make corrections. Then when I get home I can download and merge the changes into the database on my Mac.
This is by far the most exciting application I have found for my iPhone!