Just have to say, this would be a really useful option/capability, because a lot of end users are not intuitively familiar with web and/or mapping conventions, so providing tool tips would be a great way to prompt users with instructions (like informing them that greyed out layers will appear as you zoom in on the map--so many users think, "the box is checked, so why isn't the layer showing up?").
UPDATE: I just discovered that Map Viewer has a tool tip built in for the example I gave above, namely that hovering over the layer displays a tool tip saying, "Hydrants (Invisible at current scale)"; however, the web mapping application referencing that web map does not have the same functionality built in.
