[eluser]WanWizard[/eluser]
With DataMapper 1.8.2 (on CI 2.0.2) I can not reproduce that.
When I execute the exact same code, the new post record is created, and the id of the new post record is used to insert the relation to all selected categories in the categories_posts table.
You're installation is up to date? Your models are defined correctly (as in defined BOTH ways)?