It will act as the overworld like in Super Mario World, etc. The first node will essentially be Bitrim: Prologue.
Alpha 4 will be out soon. Then it will hopefully be put on Desura, advertised everywhere I can think of, with Alphas coming thick and fast afterward.
Remember: You can pre-order Bitrim by buying the Prologue version above. You'll get an offline client version (instantly) and full access to Bitrim when it's released, including alpha/beta access.