The appliance looks attractive, but hold on a sec, a combined hardware and software solution! Why on earth are they selling the hardware alongwith the software? :-)
Well, their FAQ has this to say to the question:
Why offer an appliance instead of shipping traditional enterprise software?
The Google Search Appliance delivers a simpler, more reliable solution with a lower total cost of ownership (TCO) than traditional enterprise software can offer. Because all customers run on a hardware configuration we have optimized, Google's software is more thoroughly tested. Because there's no need to run Google software on multiple operating systems, we're able to develop new features more quickly. And you get a lower TCO because there's no need to manage the complexities of the underlying operating system yourself.
I can think of some ways, how one could modify the OS (Linux) kernel to better run a search-engine. But I am very curious to know why one would want to modify the computer hardware, and what modifications they would make for optimizing it to run a search-engine.