Only time i have noticed heat is when playing 2 hours gaming session of hardware taxing game, and even then we are talking slightly warm rear panel. I haven't really noticed any heating in downloading stuff.
People are using silicone protection skins on their Vita and i haven't heard any overheating stories yet, so i don't think that cases would harm it one bit.
if you leave the screen on then yes it gets f...ing hot I didnt block it and the little strap on the small pull n go case kept touching the screen and it never went off lol --- PSN:JustArc_ Waiting for: Persona 5. Now playing:Metro: Last Light, Persona 2: Innocent Sin, Injustice: Gods Among Us.