The UI has a few subtle differences. When viewing the iBookstore from the iPhone, the icons across the bottom are Featured, Charts, Browse, Search and Purchases. On the iPad, they're Featured, New York Times selections, Top Charts and Purchases.
Bookmark synchronization is done via iTunes. When first launched, you're asked if you'd like to setup bookmark sync. A simple "yes" will do.
You can move previously-purchased iBooks to your iPhone in two ways. The first is to sync them like other items in iTunes. With your iPhone connected to iTunes, click the "Books" tab. You'll see options to sync both audio books and iBookstore purchases. You can opt to sync all books at once, or pick-and-choose individual titles.
The other method is to go into the iBookstore with your iPhone and tap Purchases. You'll see a list of all the purchases you've made with an option to re-download each one.
Reading on the iPhone is of course different than the iPad due to the screen size, but all of the settings in the iPad version, like brightness, text size, search and bookmark, are in place.
We'll have an in-depth look at this app up soon. For now, try it out and experience Apple's idea of the iPhone as an ereader.