There's two different concerns here, one is building Play with sbt 1.0 (should have no impact on users), the other is cross building the Play sbt-plugin against sbt 1.0. Off the top of my head, the following other sbt plugins will need to be cross built first (and some of them might already be cross built, I don't know):
* twirl
* sbt native packager
* sbt-web and sbt-js-engine
The whole sbt-web ecosystem will need to be cross built, for those parts of it that use sbt-web-build-base (sbt-web and sbt-js-engine both do), that should be pretty straight forward, sbt-web-build-base is like interplay, it provides an sbt plugin that provides all the common build settings, so it should be a simple matter of updating that, to support cross building against sbt 1.0, and then upgrading each project that use it