Linux Libertine comes in 2 varieties: TTF and OTF. The Wikipedia/MediaWiki CSS (checked on English and German Wikipedias) use “Linux Libertine” as the font for headings, then falling back to Georgia. I use Ubuntu 12.04, whose repositories only have the OpenType version of Libertine fonts (which makes sense as OTF format is simply better). To prevent confusion, Linux Libertine folks use a ‘O’ suffix on their OpenType offerings, that is the fonts are named as “Linux Libertine O.” So, even though I have LL fonts on my computer, the CSS forces Georgia.
Ideally, the CSS should use “Linux Libertine O”, “Linux Libertine“, Georgia, serif; as the rule. This is compatible both with legacy TTF systems (and MS Windows) as well as up-to-date OpenType systems.