This idea has been merged with Definition Query in TOC.
I was talking with my supervisor about this one, even before we knew the idea existed outside of our super exclusive circle of genius. His idea- is to have either an indication in the TOC directly (perhaps a superscripted “D” next to the layer name) or even a Definition Query tab like Source, Visible Layers, etc… that would show which layers had Def. Queries. Alternatively, developers could also add the superscripted “D” - to the layer names listed within the Visible Layers tab if the tendency is to shy away from cluttering the TOC more than absolutely necessary.
Another one of my super high tech mashups for your viewing pleasure: