IWF Not Blocking!
It appears that the IWF are not adding non-photographic "prohibited images" to their block list. Instead, they're only taking reports of such images hosted in the UK, and treating them in a similar way to extreme porn.
See item 7 (b) of their board meeting minutes from 29th September 2009: http://www.iwf.org.uk/corporate/page.215.617.htm:-
"(b) Non-photographic images (“NPI”)
The Funding Council has agreed to the inclusion of prohibited non-photographic images of children within the IWF’s remit, with its inclusion being subject to the same conditions which applied to extreme pornography. The Funding Council requested that the response to the Ministry of Justice (“MoJ”)should make it clear that the material would not be suitable for inclusion on the ‘block list’. The Board agreed that the response should include an option for the IWF to withdraw from including NPI within its remit if the conditions for its inclusion were not met and that MOJ attention should be given to the PR implications of the legislation.
Action 10: A letter to be sent to the MOJ from the Chair reporting the industry and the IWF’s willingness, subject to certain conditions being met, to include prohibited non-photographic images of children hosted in the UK within its remit."
See also the IWF report page, which now includes an item for "Non-photographic child sexual abuse images hosted in the UK": http://www.iwf.org.uk/reporting.htm.
This means we can't rely on the IWF to shield us from "prohibited images" (or extreme porn) hosted outside the UK (that's most of the world).