Regarding population the guess on the official forums is 12,000 per server. However we don't know whether numbers are throttled because of the population being so dense in the starter areas. Since 12k * 39 is almost half a million, then the EU servers probably have similar numbers to the US ones that seems a bit high, almost a million playing before the game's even launched. It must be much more than 1,000 however as some servers have had queues of well over 1,000 and presumably most of those people eventually get in. 2268 is the highest queue position I've seen reported so far.
If it's not being throttled and the servers are all close to full already that makes 39 * 12,000 players on the North American servers, 468, 000. And a similar number on the EU servers.
It certainly wouldn't surprise me if there were a million players. The statistic they've given is two-edged. They've said that a million accounts have registered for the forums and people have been quick to point out that some of them won't have bought the game. However most people who play MMOs DON'T register for forums, there may be just as many people playing without registering on the boards as there are customers who registered on the boards but didn't pre-order.