"""Simply serving files doesn't take much processing power - the bottleneck is in the ethernet cable (gigabit or no)."""
Actually I've found that on a single core Atom, the CPU is the limiting factor in network speed. That's with a 3 disk software raid5 which can read at about 120MB/s, but which yeilds only 20-25MB/s using CIFS (Windows Networking.) NFS gets between 30 and 40, and AoE can manage somewhere in the neighborhood of 55MB/s. The CPU is, of course, pegged the whole time.
I know it doesn't sound like a network file protocol should take much cpu time, which is mostly true. The issue is that between issuing the syscall to read or write and having the action complete, the kernel is locked and nothing else gets to use the CPU. That means the network file server process has a vanishingly small amount of CPU time to work with. And the problem isn't helped when two processes are reading at the same time, causing lots of seeking, and thus more io-wait cpu time loss. In Linux I've seen that a dual core helps, since only one core is totally blocked at a time, and the other is free to actually process things. I haven't tried that with an Atom system though.
Of course I have no idea how io locking works in the Windows kernel, but evidence suggests it doesn't do as well as Linux, since I could only get 7-9 MB/s running WinXP, and 6-8 with Server 2008.
I really don't think these systems are all about economy, since once you lump in a 300W psu and some drives, the difference between an Atom system, and say, something based on a mobile Core 2 (Duo) isn't terribly significant. The Core 2 would cost quite a bit more, but it'd give you a seriously more modern platform to play with.