According to the official CXL site the CXL Beta is almost upon us:

Tomorrow at 7pm GMT Beta registrations will be closed. Which means you still have a day to register at jointhebeta.citiesxl.com if you haven’t done so already.
You probably understood it by now: the Cities XL Beta will soon be upon us… Stay tuned as we will get more into the details of it in the upcoming days (including some tricks to get into it).