In a recent meeting I was asked:
I want to be able to report on what objects where changed since the last label
I am pretty certain labels are NOT in the data warehouse. The easiest method would be to create a report using the TFsVersionControl database and the table called Dbo.tbl_Label.
Unfortunately the Team Foundation Server development team has indicated they are reserving the right to change the data schema of the 7 TFS databases.
This means you cannot easily create reports using labels in a supported manner.
That being the case for a "non easy"(grin) method you could probably call a function from your report that retrieves this information like Buck is doing at: Displaying the labels on a file, including label comments