HealthVault Response Paging
There is a parameter on the HealthRecordFilter object called MaxFullItemsReturnedPerRequest and it is set to 240 by default. If your search returns more than MaxFullItemsReturnedPerRequest items then you will get full items for the first max items and then HealthRecordItemIds for the remaining items. You can then retrieve the other items by ID.
If you are using the .NET HealthVault SDK then all of this logic is handled for you. The HealthRecordItemCollection returned by GetMatchingItems will appear to contain all of the items. Attempts to work with the guts of an item that is still on the server will trigger the retrieval of that information.
(update on 10/21/2009: the default for MaxFullItemsReturnedPerRequest is 240. I originally said 30 and that is incorrect.)