When Apple, or anyone else, publishes source code on the internet, it is automatically copyrighted. Sometimes that copyrighted content includes an open source license. If so, then you can then use that code under the terms of that license.
In theory, said license could say anything. But they typically conform to one of a number of standard open source licenses. Usually, you can continue to use and re-distribute the source even if the original content is removed. Just make sure to confirm that with the license itself. Apple's licenses are typically very unobtrusive. But licenses from other sources can be very restrictive.
Unfortunately, Apple is well-known for deleting documentation, videos, and especially sample code. If they publish anything during this week's WWDC that seems like it might be useful, download and archive it. Never assume that anything Apple publishes will be available forever.