Documentation standard in our department is Docbook, and we have set up build-processes for our documentation already. Thus it would be very handy to export Docbook files directly from Confluence.
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Atlassian Update: November 10, 2011

Hi All,
Thanks for your patience on this issue. After some thought we have come to the conclusion that this is not something we would consider to be core and strategic to Confluence. This is a feature request which we are open to partners and plugin developers tackling as we don't have any intention on doing this.

One of our partners, k15t have just released their Scroll Wiki DocBook Exporter for Confluence which satisfies this issue.

Cheers,
Sherif Mansour
Product Manager