For some reason OUTER JOIN consideres fields having both NULL values as not equal (at least in MSSQL 2000). I use this as workaround:
Code:
FROM dbo.V_HEAD_DETAIL_Program AS V1
LEFT OUTER JOIN
dbo.T_ROAD_HEAD AS H1 
ON (V1.prg_lob_key = H1.RMAP_ID or V1.prg_lob_key is null 
	and H1.RMAP_ID is null)