Building low-power servers has typically meant using laptop CPUs. The main issue is that these cost a lot more than comparable desktop CPUs. At the moment, a 25W dual-core mobile CPU (Intel, Socket P) costs around 200 Euros (e.g., an Intel Core 2 Duo P8800 Penryn 2.66GHz 3MB L2 Socket P 25W costs $260 at Newegg and 222 Euros at Mindfactory). This is more than times as much as a similarly speced desktop CPU (e.g., Intel Dual-Core E5400 Wolfdale 2.7GHz 2MB L2 Socket LGA 775 65W costs $70 or 55 Euros, respectively).
This is not the only cost difference: since the processors use different sockets, different motherboards are needed. Currently, Newegg only has 2 Socket P motherboards. They cost $120 and $160, respectively. This is again a factor of 3 price difference relative to similar desktop equipment. Moreover, neither is very impressive--200 pin SO-DIMMs and a single PCI or PCI-E slot plus a mini PCI-E slot.
Another issue is that these "high performance" chips are typically not needed in a home server. In my experience, disk I/O is usually the performance bottleneck.
These three issues--power use, cost and performance--and the need to replace our MythTV & backup server motivated me to look at the Intel Atom offerings. The CPU and main board come bundled together and cost around $80-$100/65-80 Euros. The CPU has a TDP of around 10W. The main drawback is the mini ITX form factor, which means there is at most a single expansion slot. After some hunting, however, I came across the Point Of View MB-D510-MATX, which has 2 PCI slots, a PCI express slot, a mini PCI-E slot and 2 DDR2 memory slots. And, it cost just 80 Euros at PC-Pirat. FTW
I'd never heard of Point of View and there was not much info about the product online (even the manual wasn't online!). I decided to give it a go nevertheless, given the small monetary risk. I'm very pleased with the result and can recommend the product.
The board requires 4 slots of space.
I scanned the manual.