Celebrim
Legend
So that's it. Sort of a let down I suppose - there's really no "inspiration" there except some player wanted to pick some locks and the DM came up with a thief class from there. And now, the above is not speculation or bragging, as you can see on Gary's end he writers it down and mentions he did get the idea from those players at Aero Hobbies.
Very cool. Thanks for providing the information. I was going to look for it, and now, here it is all nicely laid out.
The thief, with the exception of the wand use, is pretty nearly in a complete form with all the elements present in the original publication. This suggests that Gary could not in fact know the inspiration for the thief class himself, unless it was disclosed to him in a conversation or the source material was so obvious that anyone seeing this particular collection of abilities would immediately call in mind the source material. Any musing then by Gygax as to the inspiration would be speculation by him, since he ripped the ideas off from someone else. Unless we can actually find passages in Jack of Shadows and Eyes of the Overworld that place those ideas together in the same character, it's entirely possible that it was in fact just an ad hoc, spur of the moment innovation. I believe I have both books on my shelves, but not the time to skim through them at the moment.