Look up DOI in PubMed Central

Search PubMed Central for the DOI, and display a link to the full text if it's available.

Image is from http://www.ncbi.nlm.nih.gov/pmc/ - might want to choose a better, free one (or display the link differently).

I haven't tested in Internet Explorer; there's a small possibility it might not like the XML document, but should be ok.