These beasts are far too big and heavy, with terrible battery life, to be used by photographers in the field, but for an editing station at an event like the World Cup or the Olympics, they do come into their own.
Naturally the larger screen makes editing much easier. Also these models generally have more power and can speed through tasks quicker than conventional laptops.
Lenovo (who bought out IBM's ThinkPad range) has just announced an amazing 17" laptop specifically targeted at photographers; the ThinkPad W700.
It can be specced up with an Nvidia Quadro graphics card which runs at 1920x1200 on its 17" display, but has the power and capability of driving an external 30" monitor.
It can have up to 10Gb of RAM (including 2Gb of Intel Turbo memory), have dual hard drives, a Blue Ray writer and built in CF and SD card reader. Amazingly, it also has a built in Wacom tablet and an X-Rite screen calibrator! This works when you shut the lid and calibrates the screen in around 80 seconds. With all these goodies though, the price does rise to the $6000 mark.