This still seems to be the case in 2008. The CHECK CONSTRAINT on the underlying tables must be set to "Check Existing Data on Creation or Re-Enabling = Yes." I couldn't find this anywhere in BOL, but once I changed the code below, I was no longer getting the partitioning column error.

Old Code:

ALTER TABLE [dbo].[d5match9] WITH NOCHECK ADD CONSTRAINT [CK_d5Match9_PartID] CHECK (([PartID]=(9)))

New Code:

ALTER TABLE [dbo].[d5match9] WITH CHECK ADD CONSTRAINT [CK_d5Match9_PartID] CHECK (([PartID]=(9)))