I could speculate about why the NIH study is still awaiting release, but all the information has been confusing and contradictory, so I'll hold off for now. Instead, let's look at the results of the CDC study. Did they fail to detect a link because there isn't one, or because their methods were flawed?
The cohort problem
The CDC defines ME/CFS very broadly. As a result, their previous research includes many people who probably don't have ME/CFS. Though this paper is touted as a replication study of the Whittemore Peterson Institute's research, they didn't define their population using the same criteria. Here's an excerpt (all emphasis mine):
The 1994 International CFS case definition [used by the CDC] and the Canadian Consensus Criteria [used by the Whittemore Peterson Institute] are different and do not necessarily identify similar groups of ill persons. Most notably, the Canadian Criteria include multiple abnormal physical findings such as spatial instability, ataxia, muscle weakness and fasciculation, restless leg syndrome, and tender lymphadenopathy.These are all common symptoms of ME/CFS.
According to Dr. Suzanne D. Vernon, the CDC did not replicate the WPI study's techniques:
[T]he samples from these three study cohorts were collected using different types of tubes, each of which has a distinct way of being processed. As if this weren’t bad enough, none of the blood tubes used were of the same type used in the Lombardi study. (They used tubes containing sodium heparin that are intended for use with virus isolation). The blood tubes from the 18 Georgia registry patients are designed to collect whole blood and preserve nucleic acid; it is not clear where the plasma came from for these subjects since plasma cannot be obtained using these blood tube types. So the explanation for not finding XMRV in these samples is simple – this was a study designed to not detect XMRV using a hodge-podge sample set.You can read Vernon's critique at CFIDS Association of America's site. It's worth noting that the CFIDS Association of America is regarded with suspicion among many members of the ME/CFS community, who view it as the CDC's lapdog. In other words, the criticisms that the CAA now levels at the CDC are in no way knee-jerk responses to news it didn't want to hear.
So, it seems as though the CDC paper has some serious issues. Now we need to convince the powers that be to release the NIH study. Only then will we be able to see if it holds up to similar scrutiny.