I used to have a gaming laptop, but i find that when i'm not at home, i didn't play games enough to warrant the loss in graphical power for the price.£600 in the uk gets you around a 9600 GT atm, as far as i know (could be slightly better now). But the same £600 bought me a phenom II 940 and a hd5870 desktop. I know which one i prefer
For mobility i'll soon be getting a dual core atom netbook w/ an nvidia ion chipset. It'll cover any basic gaming needs (the occasional mmorpg for example) but won't cost me the hundreds and thousands i'll need to be able to run games decently on a normal laptop. but it'll be primarily so i can use it in when in physics labs, because they seem to be using horribly old computers with linux (although they'll probably replace them the moment i buy the netbook, but that's just sods law).
But in short, laptops really aren't the best way to play games financially, really and truly. You'll have to upgrade more often to play the newest games. For example, hardly any laptops can really play gta4, and most mainstream ones will struggle with battlefield bad company 2. Whereas desktops of the same price (currently and will) breeze through them. If a desktop doesn't play it, you're looking at up to £200 to upgrade if a few parts are slow (maybe every 18 months to 2 years for mainstream parts). A laptop means spending upwards of £500-600 to get a whole new laptop (pretty much every year at least to keep up). So if you have money to burn, laptops are a great option for gaming.
Of course, it does depend on your needs, personally i just found i was playing games most of the time while my laptop was in a fixed place (i.e. my desk). Also, i like messing around with computers. But i definitely know the advantages and disadvantages of both types of computers. As a gamer, i just prefer desktops, it's cheaper, i have surround sound and a massive screen. But laptops are great for those who don't like tinkering with computers, really NEED the portability (i.e. you don't have a home internet connection, like i didn't way back when), and don't mind having half the power for double the price.