All clear plastic yellows with time. I think the iMac handles suffered a lot, I saw a lot of yellow handles on non-smokers iMacs, it's the heat buildup or something. But nicotine certainly speeds this up. Some yellow buildup can be removed by using a non-abrasive cleaning product on a damp rag (standard warnings apply about using chemicals on computers). Once the plastic itself has yellowed tho, smokers or not, you can't get it clear again. Then again Cascade makes a product for stained tupperware-- I once soaked my iMac's case in a mixture of it, and it definitely made it somewhat clearer-- but I was bored and most people won't want to dismantle their entire computer. I was thinking they might use the same chemical in the Armor-all dashboard wipes, but I've never tried them. Anyway with light smoke on an irregular basis, it should take plenty of time to be noticed... you'll probably be using a fabulous new Mactel by then 🙂