Dual-headed laptops is such a novelty and rarity that for some held back adoption of a laptop as a primary machine. Multiple displays are possible, and Matrox's DualHead2Go, which was released a while ago, is one of those toys that makes multiple external displays on a laptop feasible, easy and affordable. PCStats has a runover of DualHead2Go
along with a review. They cover the physical specifications and compatibility of this nifty device as well as the software and the quality of the display. It seems that users with embedded ATI graphics will end up having the most flexibility in their displays, but due to some inherent limitations of all embedded graphics there are many things that can't be done, such as a maximum of 2 1024x768 displays on Intel GM/GME/910GML/915GM graphics, very popular in Centrino branded notebooks.
The simplicity of the device and the apparent ease of installing make it sound like a good fit for notebook-based desktops. They have comparisons showing the space you gain, and discuss some of the flaws or visual artifacts you may encounter. If you are a notebook user and want to use multiple displays, read the review to get a better idea of what your options are.