When even 200GB laptop drives are rare and 160GB is still considered to be high end, Dell and Alienware are looking to push mobile storage further. With some of their newer model laptops, you have the configuration option to include 250GB 2.5" HDDs, a behemoth amount well above the average. An industry first, for sure, especially since they operate at 5400RPM - faster than the drives Fujitsu has made (publically) available in capacities that high.
That amount of storage definitely comes at a premium, but not any moreso than a similar machine with a 160GB drive in it. Interesting. If you want to push it even further, a few models that accept two drives, such as the XPS M2010, let you configure them with two - giving you 500GB of space in a laptop. Replacement desktops indeed.