Given what happened in Durban in 2001 at the 'UN World Conference against Racism', the movement to boycott Durban II makes pretty good sense to me. Israel and Canada are staying away from this second conference and some other countries, including France and the Netherlands, have been considering doing likewise.
At the same time, I think condemnation of the US decision to participate in the preparatory stages of Durban II are premature. The argument is that by participating the US willy-nilly legitimizes the Durban process. The counter-argument is, as you'd expect, that by engaging the US gets a voice and might be able to affect what happens. According to this report:
[T]he Americans [are] hoping that by active participation in the conference they might influence the tone of the discussions and their outcome, especially by preventing the demonstrations of hostility toward Israel rife during the first Durban conference of 2001.
Should US participation secure a good result at the conference, or if, failing that, the US pulls out, condemns the decisions taken, and so forth, this could not be construed as legitimizing a politically noxious process. So it remains to be seen.