Viewed through this lens, clustered computing is here now and has been here for a while. Only the attributes that describe it at any time have been changing. But of course, these aspects determine its practicality.
Hardware attributes relate to both the processor and the network.
The latter two have become problems. For the network the attributes are latency, bandwidth and scalability. There is also an additional attribute which is uniqueness. This affects availability and cost. We are in a time when plug and play hardware components provide ever lower cost computation.