I bought my license through CGRiver, which is actually labeled as CGWorld on that page. It's definitely a valid license, just took a little bit of time (a day maybe?) for the Pixologic team to approve my purchase and send me the install file and license number. edit: Oh and I forgot to mention, right after the payment went…
I bought ZBrush directly from Pixologic, but with a huge student discount price. I believe I had purchased it for $350-400. It was so long ago that unfortunately I cannot recall! However it does come with free upgrades. I would email customer support and ask.
So far I can pretty much confirm what EricHall said. It took about a day and a half before I could actually download it and as far as I could tell I was still mostly dealing with Pixologic. So far so good. If I ever have any issues I'll revisit this thread.
Its probably just them using the usual model for distibution; retailers sell the product cheaper than the "manufacturer"(developer, in this case). To answer your question directly though, I bought my license for zbrush 4 from Novedge and have been able to get every update Pixologic has released since so you ought not worry…