CREATE TABLE [dbo].[tbl_bulk_insert_datetime_issue] (
    [id] [int] NOT NULL,
    [description] [varchar](20) NOT NULL,
    [datetime] [datetime] NULL,
CONSTRAINT [pk_bulk_insert_datetime_issue] PRIMARY KEY CLUSTERED (
    [id] ASC
))

BULK
INSERT [dbo].[tbl_bulk_insert_datetime_issue]
FROM 'C:tempbulkinsertsample.csv'
WITH
(
    FIELDTERMINATOR=';'
)

1;row01;
2;row02;20130401
3;row03;
4;row04;20130515

id          description          datetime
----------- -------------------- -----------------------
1           row01                NULL
2           row02                2013-04-01 00:00:00.000
3           row03                NULL
4           row04                2013-05-15 00:00:00.000

1;row01;
2;row02;20130401
3;row03;not_a_datetime
4;row04;20130515

id          description          datetime
----------- -------------------- -----------------------
1           row01                NULL
2           row02                2013-04-01 00:00:00.000
3           row03                2013-04-01 00:00:00.000
4           row04                2013-05-15 00:00:00.000

1;row01;not_a_datetime

id          description          datetime
----------- -------------------- -----------------------
1           row01                1900-01-01 00:00:00.000

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

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

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).