In July we enabled protocol-relative URLs on testwiki, and asked for bug reports. We did this in preparation for native HTTPS support for the sites. We received and fixed a number of protocol-relative related bugs, and then tested on a few of the larger wikis. We are now at a point where protocol-relative URL support is stable enough to enable it on all wikis, so today we’ve enabled it.
For information about what protocol-relative URLs are, why they are needed, and how it’ll affect you, see the post written in July. In brief: this changes most links we output in our content from looking like http://www.example.com to //www.example.com . The change shouldn’t affect you.
EDIT Sep 28 14:29 UTC: Because of reported breakage in iOS clients, the API’s action=parse interface has been hacked not to return protocol-relative URLs. This is a temporary hack that you should not rely on; fix your clients instead. For details, see http://lists.wikimedia.org/pipermail/mediawiki-api-announce/2011-September/000024.html