This item editor would be for creation of out-of-game resources that you can use in quests, research, whatever? I dunno, we'll have to see I guess.
Yes and no. Say I make a new skin and texture for basic Chain Mail. Now I want to see how my created Chain Mail Skin will look applied to the chain mail and worn by a soldier. If the "Unit Designer" was available to me without me having to get in game, I can fire up the designer and design a unit wearing the chain mail. Now I can see what my re-skinned chain mail looks like quickly and easily. If it doesn't work like that then I either have to have the proper converters to pull up the in game model in 3DsMax or I have to fire up the game, found a city, play until I have the tech to make chain mail, then design a unit wearing it, all so I could see what a skin looks like when applied to a model. See what I'm getting at now?
Since there's a tile editor and all those pieces, if I were to re-skin those pieces, all I'd have to do to see my changed skins is fire up the tile editor. A unit wearing chain mail though isn't really classified as a "tile" nor as a "item" I don't think. If we get a "Item editor" though as you're thinking then perhaps I can simply fire up the editor and it will show me what the item (being chain mail in this case) will look like, whether on a soldier or not.
I know a week or two ago someone brought up "item creation" and Frogboy distinctly said that the "Master Forge" was something they kicking around for an expansion idea. Honestly this leads me to think we wouldn't get a visual item editor until then (which I think is a bad idea considering everything else we'll be able to mod from the start). I guess we'll just have to wait and see how it works first. I could be wondering/worrying for nothing.