Right before the outbreak of the war in South Ossetia, a respectable private intelligence agency, Stratfor*, wrote about the PKK's attack on the Baku-Tbilisi-Ceyhan oil pipeline earlier that day. It notes all the reasons why it seems unlikely that the PKK ordered and carried out the attack by themselves (emphasis mine):
Although the PKK has explicitly threatened attacks on the pipeline since 2005, the group has focused its attacks instead on a natural gas pipeline between Iran and Turkey that runs straight through some of southeastern Turkey’s most volatile areas, making it far more vulnerable to attacks than the BTC pipeline. For the PKK to carry out an attack on the BTC, the group would have to expand beyond its usual area of operations. That could prove difficult at the present time, however, considering the amount of pressure the militant group has been facing since Iraq’s Kurdistan Regional Government began cooperating with Turkey and the United States in early 2008 in uprooting PKK safe havens in northern Iraq. But the PKK also has a desire to make a strong comeback after spending the last several months under pressure from Turkish forces, and striking the BTC pipeline would certainly be a coup for the group. The PKK also might look to its former allies in Moscow for support at a time when Russia is seizing upon every opportunity to use energy resources to apply political pressure on Europe.
Such attacks would not come without risks, however. A “success” would likely be immediately followed by a military crackdown the likes of which the Kurds have not suffered since the height of Turkish-PKK clashes in the 1990s.
This article was published before anyone knew how the Russian-Georgian conflict was going to play out. But in retrospect, it happened right when Moscow would have wanted it to happen. Two crisis events – the PKK bombing and a Russian invasion of Georgia – would stick in Western investors minds, and make them seriously think twice about the risk inherent in building new pipelines through Georgia to bypass Russia. And by using the PKK (which had ties with the USSR) as a "subcontractor" Russia doesn't risk raising the ire of the West.
* The site is subscription-only, but it allows you to see articles if you click a link from Google. So, do a search for the title of the article and click on the link from Google's search results page. Sorry, Stratfor!