As you might have already noticed, the BackGroundWorker class is missing in Windows RT / app development.
The BackgroundWorker was actually just a wrapper for the following class:
So the fun begins again with Threading.
The ThreadPool for Win RT is well documented in the Windows Store Apps Development Center:
For those, who like to actually work with an emulated BackgroundWorker, there are also some folks in the web who did build a custom version of the BackgroundWorker like: