Maybe it's different in the UK. But my friend works at the Apple store and he says they don't even find out until the morning. They have to arrive early at the shop, and on the day of a new Mac launch all the advertisements/products are in a sealed box and they are given around 10 minutes before opening to get the information, which lets face it they don't need to know much about it, if we assume the Bloomberg article all they have to say is; Retina, slimmer, ivybridge and SSD. I don't know if this true for all stores, but certainly true of the one my friend works at. The only thing he can comment on is inventory levels of a particular shop, but he says that it varies so much it's impossible to truly give definitive answers.
Even then, if we assume it is going to be launched immediately at WWDC, it be in store the next day, which has given employees the day before to find out about it via WWDC.