I really liked the Belkin Grip Vue, clear, but it yellowed so bad I got rid of it. Emailed Belkin about it, but they ignored me. Won't buy from them again. Here's the old Belkin on top of the new clear case I bought.
Another yellow one here, it has been put away clean in a drawer for 75% of the time so it's not a sunlight / daylight / heat problem. I have another case I prefer for general use..
the exact same case without their name on it yellows too... generic tpu clear case.. typical.
fwiw, the belkin iphone case my buddy has is also yellowing.. so it's clearly the material being used.