For specific details and criticisms, see:
Talking Points Memo (February 14, 2007)
TPM Muckraker (February 14, 2007)
Outside the Beltway (February 12, 2007)
The Pentagon's briefing PowerPoint on the EFPs can be found here. Interesting that the labels on the weapons shown in photographs include English wording, but that's not a sign that they weren't made in Iran, but only a consequence of the fact that English is the lingua franca of the arms trade.
UPDATE (February 27, 2007): A factory producing EFPs has been captured in Southern Iraq--and the parts that have identifiable origins did not come from Iran.