Obviously gaming on laptops drastically reduce their lifespan
I wouldn't say this for anything but the battery specifically. As far as the rest of the hardware goes laptops have a problem with dust after a year or so, being so cramped inside, which contributes to overheating. This can be fixed with judicious use of compressed air.
But unless you're just pushing the thing transistor-meltingly hard with games on highest settings, I can't imagine it'd be a determent.

But I'm biased, having just spent more money than I care to admit on a laptop of my own to replace my desktop. I don't plan on gaming on it though, which is half the reason I made the switch in the first place. I have a mixed opinion on laptops in general too. One Sony Vaio laptop not only lasted for over 5 years of use, but also survived a 15 foot fall off the top of some stairs (well the screen broke, but it still got use with an external monitor for years after that). Then one HP Pavilion laptop just up and died one day after less than a year of normal light use, got sent in for repairs under warranty, and then up and died again a month later. Consequently stay far away from HP and their ilk. They saturate the market, but it's a bad decision to buy from them.
I have high hopes for the Lenovo X220 I've ordered. 7 hour battery life normally, i5 2520M CPU. Not for gaming, but it'll hold down Morrowind just fine I think.