I came across a bug when using the search box on Pubmed Central. When you search on http://www.pubmedcentral.nih.gov/, you end up on http://www.ncbi.nlm.nih.gov/sites/entrez. This latter site is already recognized by the NCBI Pubmed translator, but Zotero doesn't import the search results correctly.

For instance, the query "maris geertman pronk dijken" yields a single paper:

http://www.ncbi.nlm.nih.gov/sites/entrez?term=maris%20geertman%20pronk%20dijken&search=Find%20Articles&db=pmc&cmd=search

This paper has both a PubMed Central ID (321313) and a PubMed ID (14711638). The translator recognizes the paper, but when you import it, you end up with a different paper in your Zotero database, namely the one with 321313 as PubMed ID. This bug can probably be fixed by scraping the PubMed ID on the Pubmed Central search results page instead of the PubMed Central ID.