What would be the copyright implications of using AD&D monsters in a 3.5 monster generator, does any one know?
Reading through some of them bought back some fond memories.
That's a tricky question. Unfortunately, I wasn't around when a lot of this was worked out, and I'm no kind of lawyer, so keep that in mind.
The way I understand it, the CC and Necromancer Games (for the Tome of Horrors) had an agreement or agreements with WotC (again, the Necro ToH designers and the CC crowd at the time were largely the same crew, so this isn't clear to me) to convert old monsters. I don't know if this was a formal contract per se or more of a "gentleman's agreement"/verbal contract. I also don't quite know the terms of that agreement; the monsters in the ToH are OGC and usable by anyone under the terms of the OGL. That should also be true of the CC, at least if the same agreement with WotC is in force and assuming the CC's section 15 is correct. So anyway, I guess you can use anything in the ToH under the terms of the OGL.
It's also possible that no special agreement with WotC was needed. One thing we've heard a lot recently is that game mechanics can't be copyrighted in the US, and the 3.X game rules used in these conversions are OGC anyway (for ex, the CC uses only mechanics from the SRD and stuff we make up ourselves). But some of the monster names could be copyrighted (we rewrite fluff, so that should be ok), I don't know.
Also, back in the early days of 3e, WotC released "conversion guidelines" aimed at protecting their trademarks and copyrights on adventures. This required use of the d20 STL, which has now been revoked (the way I read it). But it's not clear that this applies to monsters if you don't use the copyrighted text; the names would appear to be the big issue. I have done conversions of some older edition monsters and not heard peep from WotC (I suspect I'm way under their radar), as have other fansites (some of my work is on Candlekeep along with others). ENWorld member Garnfellow has conversions from White Dwarf up, but he doesn't think they can be made OGC. So it's murky. And the CC won't offiicially post anything from non-WotC sources, like Mayfair Role-Aids, even though we're doing the conversions.
So, all in all, it's iffy. The only thing I can say for sure is that ToH monsters are OGC -- even Paizo has used them.