I have this mysterious grid on my art board that appeared because I hit something accidentally. It is a mesh grid for some purpose. No clue what it is or how to get rid of it, let alone describe it. Remains after I turn the visibility of all my layers off. No selection tool will select it. Unknown cubic icon in upper left of art board. No apparent cues in Illustrator about what this is. Nothing in appearance panel. Lovely program, Adobe. I just upgraded from CS3 to CS6; not a feature in CS3.
The only solution I know is to create an entirely new document and cut and past may entire illustration into it. If I could post a screen shot, I would show you. The solution is would obviously be trivial if I knew where to look. -j
Its the perspective grid and you hit Command or Control + Shift + I if you hit those keys again depending on which platform then it will go away.
CS5 Features : http://tv.adobe.com/show/illustrator-cs5-feature-tour/
You might want to start with those.