I don't have an ipad, but I do use some apps on my Droid with photography so can address some of the appliance, what is it good for, questions.
First up would be as a GPS track logger with My Tracks app. Saves tracks to the phones SD card or can push them up to google docs or google maps.
To make syncing tracks to photos easier the first photo would be of the droid running the UTC Time app so that you can get the time offset between the camera and the droid.
Another app that is nice to have is GPS Status. You check this app to verify that you are tracking the GPS satellites and collecting log points. I usually wait at the trail head until I get confirmation I'm tracking.
If you are into night photography another app that is nice to have is Shutter Speed. Meter the scene at a high ISO and fast lens, then use this app to give you the shutter speed needed at a low ISO and smaller aperture.
The example below is the scene was metered at f2.8 and ISO 800 and the camera gave a shutter speed of 1 second and these values are entered on the left. On the right, you enter the ISO and aperture you want to use in the shot and the app gives you the shutter speed needed, 4 minutes and 16 seconds.
If you are out in the dark, a flash light will be handy, so you might like the DroidLight app.
Another useful app to have is the DOF Calculator. It has a sensor setting for the K200D, K110D, K100D, K100Dsuper and a setting for the K2000, K20D, and K10D.
Smartpark, or Carr Matey, ... Know where your car is parked and more importantly how much time is left on the meter.
There is the camera on the Droid and its auto GPS embedding if wanted is kind of nice to have when you want to come back to a spot. Just take a photo with the Droid of the scene and location data and why you want to come back is captured. Why not just take the photo on the camera? Why use up space on the SD card that could be used for a "real" photo, and why not know you have the location without having to rely on the external track merging software?
Photoshop.com Mobile
Trapster: Your mobile phone alerts you as you approach police speed traps, red light and speed cameras, via spoken warnings in fun voices! Join world's largest driver's social network, 5 million users and growing! Drivers helping drivers!
Sunrise and sunset information as well as lunar information.
These are just some of the things I've found. There are going to be countless others. As for the size of the Ipad in general, I think it would be pretty nice to have the above types of apps, as well as an interface that can be used to read ebooks, be a bit better for viewing photos, easier to web surf, and in general being easier to lug around than a laptop. YMMV.
Thank you
Russell