We got it fixed.  The issue has existed since the beginning.  Looking through our exception logs from PracticeStudio, several of our customers end up getting the issue on a routine basis.  There was never any method to the madness of when the exception would occur, though.  The problem was exactly like you said, we were creating a new connection and opening it in the DbDataSourceItem.ExecuteReader() method that was a local variable so it would simply go out of scope.  That made it rather hard to track.  The connection was used when you weren't on a transaction.  It would be properly cleaned up at the end of the execution.  But if you were using a transaction, the variable would go out of scope and would have to rely on the garbage collection to clean it up.  That made the error rather random.  If your .NET environment was GC'ing regularly, then the connections would be removed and returned to the pool without issue.  But, if your application wasn't needing GCs then the pool would be consumed.  We did finally find a scenario that allowed us to chew up the pool and reproduce the problem consistently, so we know that the issue is resolved.

Edit: bchase (deleted attachments)