So what's the deal with Criterion's new edition of Salò missing a few seconds of footage as opposed to the BFI UK version? The whole story is far more interesting than you might imagine. My original theory was a permissions issue with the poem, but apparently the sequence doesn't appear in the original Italian vault-master interpositive at all. The link has video clips from both Criterion's edition and what appears to be the Region 2 PAL BFI edition, and even despite the tiny window sizes you can still see a marked difference in quality.
I should also point out that the BFI is bringing out its own 2-disc Blu-ray edition (and conventional DVD) of the film, with markedly different extras than the Criterion edition.