Ugh. Do we really need this? It's the kind of feature where the idea seems better than the reality. The vast majority of users would never bother with it.
The iPad needs multi-user support much more desperately.
Multi-app/split screen could be used in a business or school environment. Watching a video for a class, while taking notes in a different app would be great. Working on a word document and pulling information from a PDF, Excel spreadsheet, or possibly two word docs is very believable in those environments.
Also, I feel a multi-user would be most accepted in a school environment where a single iPad may be used across multiple classes and students.
I personally, know very few people who would want multi-user for personal reasons. Not many of us want to "share" our iDevice since we usually would be using them at the same time. I know my kids are usually doing homework at the same time and both are using their iPads to look up information while doing it. A single iPad would just cause a ton of arguments.