The short story is, it's a royal pain in the neck.
The only "real" way of doing it, is to plug the thing into a computer (pc or mac)
On a mac, the easiest way is to plug it in and open iPhoto and import as though it's a camera
On a PC, plug it in, open My Computer and the iPad storage exists as a mass storage device you can access
BUT... either way, you only have access to the camera roll, not the photo library
If there are photos in the photo library and for some reason they don't exist on any other device, I'm sorry but that's the way it's gunna stay. You cannot export them. You can only syncronize your photos using iTunes, and this doesn't take new photos out of the iPad photo library and put them on your computer - it only works the other way round, taking new photos off your computer and uploading them to the iPad.
Why Apple?! WHY?!
If the photos were taken with the iPad, copied to it via the camera connection kit, or saved from emails/websites etc, then it's relatively easy : http://support.apple.com/kb/HT4083
If the photos were originally synced from a computer then there are third-party apps such as Simple Transfer (which has a free lite version called Simple Photo & Video Transfer) which can copy photos off via your wifi network. But as photos are 'optimised' when they are synced to the iPad, any that you then copy back to a computer may not be exactly the same as they originally were on the computer.
You can transfer photos that were originally synced from a computer via third-party apps. But as photos are 'optimised' when they are synced to an iPad, you don't really gain anything by copying them back to a computer, you are better off using a flash-drive/external drive/email to copy them to another computer - the iPad will effectively just contain 'edited' versions of the photos.