After a couple of rounds of discussion, the TSC has come up with a list of user stories to form the basis of the implementation of case management, a/k/a longitudinal data collection:
The general use case here is to provide the ability to perform multiple reports on a single thing (or "entity") over an extended period of time, so for example multiple case records on a patient, or multiple census visits to the same village.
If this is something that you would like to be able to do with ODK, please take a look at the user stories, and see if they cover your requirements. If not, let us know, either by commenting on the doc, or by replying to this topic.
This is a pretty big new feature, and it's important that we know exactly what it is that everyone needs before we start working on it, so please help us out by telling us what you would like to use it for!