Ancestry.com has released "Tree To G0" for the iPhone and iTouch, and it's exactly what I've been looking for.
The app allows you to log in to your Ancestry.com account and access the family trees you have created on their site. As a FREE app, that alone would be a reason to rejoice; but it gets better...
This app is built for more than reference. You can add individuals to your tree. You can add events or edit existing ones. If you are showing the app to a cousin, and you realize that person has never answered your pleas for a photo, you can take one on the spot, and add it to your database.
Although I saw no indication this was happening, the data is synced back to your online tree, and will show up the next time you log in there.
It also seems to store your trees on the phone, because I didn't have to wait to download a tree again after viewing a different tree.
The first screen after you log in displays your trees. Selecting one sends the app online to download the data for that tree. (If you have a large tree, I wouldn't recommend doing this without a wi-fi connection.) After a brief wait, you will see an alphabetical list of people in the tree (complete with photo, if available). Touching a name takes you to an abreviated version of the online profile page, with photo and vital events. Immediate family is shown in list form (again alphabetically).
The best thing to report about this app is that it works as you think it should. Touch a name, and you go to the profile. Swipe upwards, and a long list scrolls. Select "Edit" and a standard iPhone keyboard appears.
The only thing I wanted that is there is a tree view. Given the size of the screen, though, I can understand such a limitation. Now if the screen were just a little bigger, say, tablet-sized....
I haven't put this app through a rigorous testing, but what I've seen, I've liked a lot. Nice job, Ancestry!