There is an increasing discussion about an “ISO 26000 Assessment” mid 2011 onwards. This site and its sub-sites clarify the basics and recommend an action line.
there is no ISO 26000 certification, contractual use etc.
So, since an assessment would also need to be based on requirements,
there is no ISO 26000 assessment either.
More detailed deliberations include a description of the basic differences between certification and assessment, both performed by so-called "third parties", and a proposal for a way forward, which is perfectly in line with the letter and intent of ISO 26000: by using the 26k-issue-tool.
This way forward is called
in English self-analysis,
in French auto-évaluation,
in Spanish auto-evaluación, and
in German Selbstanalyse
Key to self-assessment is the 26k-issue-tool.