I just found a really intersting thing about concurrent users to active users to "named users", aka accounts, ratio on IBM. Not sure if it really applies to MMOs, and I would certainly say that ratios are not completely correct for a launch, but it said the ratio is for every 1 concurrent user there are 10 active users and for every 10 active users there are 100 accounts. So it's 1:10:100 for concurrent:active:accounts. I would say that means there could conceivably be almost 2 million active users. Which is kinda insane to think about.

NOTE: As I said before I don't believe those ratios are completely correct for launch but I do think they could be correct for an average day for MMOs.