Excellent question and some great answers here. Most of the ARM designs are similarly in SOC's that are targeted to a fairly integrated space (like a phone or tablet) so getting the server peripherals on them is painful. A good example of that is the RasPi which does everything through USB basically and attaching storage via USB isn't really great.

That said, there is growing interest in dis-aggregation (think of it like component servers) and there are some interesting changes that brings into the space. Reasonably soon we could see an ARM SOC with a couple of nice network ports on it, and a "rack" where part of the rack was a shared storage pool for the rack and the rest was a swarm of compute. In applications where there is a high data to compute ratio (think digging though multi-petabyte date sets for stuff) The channel bandwidth benefit of many replicated compute nodes gives it an advantage over the larger 'server' type nodes. Fun times to design distributed architectures.