I once built a system on laptops for use in critical on-site oil spill response operations (so billions plus environmental damage at stake if it doesn't work.) We used laptops because we needed the portability, so even the critical servers were running on laptops.
It turned out that in actual tests, the laptops were several times as reliable as actual server-class PCs, much cheaper, and came with built-in UPS. Server hardware is a racket, and if you have any redundancy at all built into your system, not really needed or desirable. I've used redundant/clustered laptops or netbooks as servers in quite a few projects since then - they're just better than server-class hardware most of the time.
It turned out that in actual tests, the laptops were several times as reliable as actual server-class PCs, much cheaper, and came with built-in UPS. Server hardware is a racket, and if you have any redundancy at all built into your system, not really needed or desirable. I've used redundant/clustered laptops or netbooks as servers in quite a few projects since then - they're just better than server-class hardware most of the time.