New Answer (May 2009, although this was released a few months ago):
We now call this feature "Related Items." We have added support to the base thing type which understands how to link to the GUIDs of other specific HealthRecordItem instances. For more information and some sample code, check out Eric Gunnerson's tutorial post on Related Items.
Old Answer (Oct 2007):
We’ve discussed this and determined that this is not a direction we want to go in. Referential integrity could become a very thorny issue, especially when different apps are working with the same data.
Most items can be correlated beause they are tied to a single user and have timestamps. It would also be possible to use the item extension (custom XML) to add this sort of information.
Please let us know what sort of scenario you're thinking of.