This blog post lists several apps to use for mobile grading and keeping a grade book on an iPad. It is important to note that some Student Information Systems - such as Pearson's PowerSchool - have developed apps that integrate directly with the computer-based grade books.
This product from the School Improvement Network facilitates administrative classroom walkthroughs in that it leverages the camera capabilities of the iPad/iPhone/iPod Touch and integrates it with assessment tools designed to give teachers constructive feedback.