You can find many resources and discussions about the iPad and iPad apps in education on these websites:

iPads in Education
Read about iPad school trials and reviews
iPads for Education
15 iPad Business Management apps
10 iPad Business apps
Apps for English teachers
Spartan Guides - extensive information and resources for iPads
Apps in Education (apps for subject areas)
Teach with your iPad Bloom's Taxonomy with Apps
Learning and teaching with iPads Catholic Education Diocese of Parramatta

Potential of music apps on iPad (and iPhone) - Stuart Collidge
How to use GarageBand app on iPad

Higher order thinking is not what the students were consciously involved in while playing with iPad app games, but it’s what they were in fact doing. I gave my iPad to some of the boys at school one day and suddenly there was a small crowd standing around the player, intently involved and offering suggestions. I asked to film a small group of these boys demonstrating a few of the games. They were self conscious and so the natural banter and collaboration is lost but the demonstrations still stand. You can almost see the thinking process in action.

Glowpuzzle - Problem solving game
Unblock me - Problem solving game

Labyrinth - Problem solving game
Rafter (Physics game)

"The Elements" ebook for iPad (CHEMISTRY) -
the making of the app
Review of the app "The Elements"

ART: Brushes app - Finger painting on the iPad with live model David Kassan
The free MoMA Abstract Expressionism in NY iPad app

Sketchbook Pro (alternative app to Brushes)

MATHS - conversion app - Convert Units.
Thanks to Charlie for this. Sorry about visual quality.
MATHS - Omnigraph sketcher for iPad

ENGLISH/LITERATURE - Shakespeare in Bits:

Romeo & Juliet
FILM MAKING - Reel Director (very easy and quick)

ASTRONOMY - Star Walk is a beautiful, interactive Astronomyapp giving a Planetarium-like experience. Read more here.

Psychology - 3D Brain
Another 3D Brain tutorial

Al Gore's Our Choice - eBook as App - Read more about it here.
Our Choice melds the vice president’s narrative with photography, interactive graphics, animations, and more than an hour of engrossing documentary footage. A new, groundbreaking interface allows you to experience that content seamlessly. Pick up and explore anything you see in the book; zoom out to the visual table of contents and quickly browse though the chapters; reach in and explore data-rich interactive graphics.