New research was published by Albert Menkveld. In a recent study Menkveld looked at Nasdaq trades "through the microscope" with nanosecond precision. Zooming in on trades that arrive in sub-millisecond clusters the study suggests HFTs were replenishing the order book after non-HFT price quotes were consumed, possibly sourcing them from another market.
The analysis was done on a very limited data-set, and a more extensive study does make sense. This limited study does demonstrate that nanosecond data can be processed meaningfully by organizations that do not have the resources to deal with big-data challenges, such as regulators. This in itself is an interesting observation.
Menkveld's blog post can be found here.