Maybe I've missed something because I'm coming late to this, but it surprises me that people do not seem concerned about keeping track of licenses (not just credit) for the data we'll host.
At least in the EU, recently compiled or modified databases are protected by database rights. Maybe the WMF is not affected by such restrictions, and only needs to respect U.S. law. But many Wikidata editors do live in the EU, and so do many potential re-users of the data, in other Wikimedia projects and beyond. I'm concerned that failing to keep track of data licensing will put them at unnecessary legal risk, and will hinder Wikimedia's wider mission of providing free educational material.
For instance, once Wikidata content is incorporated into Wikipedia articles, projects like Wikipedia CD Selection (http://en.wikipedia.org/wiki/Wikipedia:Wikipedia_CD_Selection) might not be able to freely release their DVD of selected Wikipedia articles in the EU unless it respects the licenses under which the data has been released. (Perhaps this is the case already, but Wikidata will make it much more visible to data owners.) We will find it very difficult to respect these licenses if we don't keep track of them from the beginning.
Recording the source of the data is a vital part of this, but it isn't sufficient. For instance, U.K. government data is often released under the Open Government License (http://www.nationalarchives.gov.uk/doc/open-government-licence/), which requires not just credit but (where possible) a link to the license itself.
I'm aware that many people feel database rights are a bad idea (including me). But they are part of the law in many countries, including Hungary (where phase 2 will be launched), so we shouldn't just ignore them.
Going back to the original question, I think the database of interwiki links in Wikipedia is unlikely to creative enough to be copyrightable. The CC-BY-SA-3.0-unported license used on Wikipedia is silent about database rights, so databases that are not copyrighted do not trigger the BY-SA requirements of the license. So I believe Wikidata's current work on interwiki links doesn't violate CC-BY-SA.