No, laptops are not good platforms for even the most basic of video encoding.
These days laptops are kind of squeezed between netbooks and tablets, but they are NOT just portable desktops. They are not good gaming systems, nor are they good media encoding/editing systems. Laptops are for when you need more freedom than a tablet will provide, and more power than a netbook will provide. One example might be that you have a fairly complex Excel spreadsheet that you need to use. A tablet can't even run a spreadsheet program like Excel, and performing a number of complex calculations on a netbook would probably be excruciatingly slow. So if there's a need for portability as well as power (within reason), that's where a laptop fits in. Not quite as clear cut as a few years ago when laptops were the only option for portability, but in a few more years things should have sorted themselves. Laptops and/or netbooks will likely get squeezed out by smartphones and tablets.
Anyway... Laptops always have the problem of heat buildup. There's considerably less space for that heat to go compared to a desktop, and also less room for fans to move that heat out. You start running programs like video encoders, which keep the CPU running at full speed for extended periods of time, and you're going to generate a lot of heat, which will ultimately have a dramatic (and negative) effect on the life of the laptop. Just take a look at some of the HP systems where they fit desktop CPUs into laptops and the heat they generated was literally melting the case. Or Toshiba's Qossimo line, where they sandwiched two high end video cards one on top of the other, and the result is probably at least a third of them are suffering from massive video failure.
Apple's designers may be better than the industry average, but they're no miracle workers. They can't suspend basic laws of physics, just distort people's sense of reality.
You want to do video encoding, get a desktop. Light to moderate encoding you could get away with an iMac, but if you're doing heavy encoding, I wouldn't take any chances, and go with a Mac Pro.