Hi and Welcome to DreamHost :)
Regarding activation time for new accounts; This seems to vary quite a bit, some have their accounts automatically approved almost immediately, others get flagged for manual approval, which can take a few days.
As for the reason why some accounts get flagged for manual approval, it is hard to say exactly, but I imagine the DreamHost FRAUDINATOR fraud detection system has something to do with it.
Of-course, if your account does get flagged for manual approval, it doesn't mean that DreamHost thinks you are out to defraud them, it just means that something about your application triggered the above automated system.
Mark