It looks like the `dx.doi.org` subdomain has been deprecated in favor of `doi.org`:

http://www.doi.org/factsheets/DOI_PURL.html
"... the DOI System uses a proxy server, doi.org (or dx.doi.org, an earlier syntax which continues to be supported)"

http://www.doi.org/doi_handbook/3_Resolution.html 
"resolve DOI names that are structured to use the DOI system Proxy Server (http://doi.org (preferred) or http://dx.doi.org)"