Why does SQL Server 2008 R2 insert an invalid datetime when using bulk insert?
On SQL Server 2005, and SQL Server 2012, I did the following:

USE tempdb;
CREATE TABLE ImpTest
(
    ImpTestID INT NULL
    , ImpTestDate DATETIME NULL
);
GO
BULK INSERT ImpTest FROM 'C:\SQLServer\ImportTest.txt' WITH (FIELDTERMINATOR=',');
SELECT * FROM ImpTest;
DROP TABLE ImpTest;


With the following data:

1,
2,20130406
3,NOT_DATE
4,
5,20130409


On both servers, I received the following:

Msg 4864, Level 16, State 1, Line 1
Bulk load data conversion error (type mismatch or invalid character for the specified
codepage) for row 3, column 2 (ImpTestDate).


Perhaps, as Aaron noted, this is a bug that needs to be addressed through http://connect.microsoft.com

I'm wondering about some of the details of your system, including locale settings, collations, physical format of the import file (is it from a Unix system? etc).