I did get Jeff's method to work in SQL Server Management Studio with the following code:

SELECT(STUFF((SELECT ', '+AS1.Source

FROM Agent_Source AS1 INNER JOIN

Agent_Source_Release ASR ON ASR.Agent_Source_ID = AS1.Agent_Source_ID

WHERE (Release_ID = @Release_ID)

FOR XML PATH('')),1,2,''))

However, when I moved the query into the SSRS dataset it wants me to declare the @Release_ID variable. SSRS has already created this variable for me but it continues to say it's not there.

I think the real problem is with the FOR XML PATH method. With even a simple select query I get the following error as soon as I add "FOR XML PATH('')" to the end of a statement:

"Unable to parse query text."