One of the great benefits of the publicity surrounding Wikileaks is that it's encouraging people to look into issues they previously might not have considered.

That said, the role of the oil companies in the Niger Delta unrest isn't exactly unknown, not many people labour under the delusion the Nigerian government isn't to some extent corrupt, and the page you linked to isn't even a particularly damning one, as governments do have a legitimate interest in monitoring activities of rebel groups and the effects on multinationals they target. The smoking gun would be finding documents suggesting that events like the Odi massacre were planned (and possible oil company collusion) http://en.wikipedia.org/wiki/Odi_massacre

What's been released recently seems to be the sort of thing that might cause discomfort to a few diplomats who've made rather unflattering and speculative (but in many cases justified) comments about their host countries but it's less than earth-shattering. The half-hearted attempts to discredit Assange feels like an attempt to appease the leaders they've privately insulted; perhpas they have more reason to be concerned about what hasn't been released.

Governments have willingly released far more inadvertently damaging material... http://en.wikipedia.org/wiki/Dodgy_Dossier