S'mon said:
AFAIK EN World's conversion library does claim to use the OGL (and is thus bound by it) though I could be wrong.
If I remember correctly, part of the problem with conversions was that people were copying the whole adventure & making it available for free. Updating a troll's stat block from a published adventure was, and is, fine; detailing the troll's lair, motivations, treasure, etc, etc, was going too far.
EN World's conversion library exists solely to update WotC's older material to 3e stats; it lives and dies on WotC's property.
The d20 NPC Wiki does not. It's a shame we can't go fully OGL (is it possible to run an OGL-compliant wiki somewhere?), but in the meantime, it -should- be allowable as a fan-based resource comparable to Canonfire! or the other "TSR's campaign-setting"-based websites out there. It's important that contributors not copy significant text directly from WotC's rulebooks (ie, entire feats, classes, or spells), and the use of 3rd-party publisher's material is probably best avoided unless scrupulously marked (this is obviously not the case for M&M or AE npcs, obviously), but there really is some space out there for the wiki to exist in.
IMO.
Personally, I love the OGL and run an OGL-compliant game, so most of my NPCs are, and will be, fairly generic and based off of the SRD. (Plus, I've tinkered with the core classes and messed with feat progression (1 per level, baybee!), so notable NPCs are kinda...different.)
Cheers
Nell.