I’ve checked out the iPhone (and iPod touch) app recently released by Ancestry.com. The free app is called Tree To Go. It enables you to examine and update the trees you have on Ancestry.com through your iPhone—no, you can’t search the databases at Ancestry.com through the app. A synchronization feature keeps the data in your phone and on Ancestry.com, well, in sync.
I’ve read some consumer reviews that complain of crashes with large trees, but haven’t experienced that myself. It seems to be a problem when the number of ancestors is in the multiple thousands.
Here are some screen captures to give you a flavor for what it does (click on an image to see a full-sized version):