Postgenomic aggregates posts from scientific weblogs and detects which papers or stories are referenced. The results can be queried through an API - this example returns all the weblog posts that reference a particular paper, identified by its DOI:
http://www.postgenomic.com/api.php?type=post&citing_paper=doi:{YOUR_DOI}&format=json
Output formats include JSON and Atom. You can also use pmid:{YOUR_PMID} to identify a paper by its PubMed identifier.