I used IPB as my forum for a few good years and there were an absolute ton of mods to add to the forum. One of which was an auto PM which like DC said.. basically sends out a pm to all new registrants. It took only a few minutes to setup and you've got all the freedom of customizing what you put in the message. So why not welcome them, point out a few quick links such as the FAQ and then let them go on. Guaranteed some of the serious people will take their time to read even a few threads before posting. I wouldn't fill the message with a ton of stuff, don't need to get overloaded right away.. but something to direct people to the basics.
I haven't searched for such a mod/add-on yet for vbulletin but im sure its out there.