DOI resolution no longer requires the dx subdomain. For example, http://dx.doi.org/10.15363/thinklab.4 can be shortened to http://doi.org/10.15363/thinklab.4. The DOI documentation indicates the short method is preferred:
Users may resolve DOI names that are structured to use the DOI system Proxy Server (http://doi.org (preferred) or http://dx.doi.org). The resolution of the DOI name in this case depends on the use of URL syntax: the example DOI name doi:10.10.123/456 would be resolved from the address: "http://doi.org/10.123/456". Any standard browser encountering a DOI name in this form will be able to resolve it. The proxy service (both doi.org and dx.doi.org) is accessible over IPv6, and supports DNSSEC. The proxy servers respond to HTTPS as well as HTTP requests.
I have begun switching to the short format and suggest Thinklab does as well. Some community projects such as PeerJ/paper-now have already migrated.
Thanks Daniel. We've updated things to use the shorter URL. Please keep the feedback coming..