It's even unfairer on the newbies (and everyone else) when MNers who are bored of seeing the same topics coming up either yell at you to check the archives, or turn the thread into a mickey take because they've been there and done that. It's not very welcoming - which is why I think that telling people that the archive is there right from the start might be a good idea.
Something mentioned on another thread this evening (I think) was a FAQ section. I agree with the SN mums who were talking about it. It would be very useful, especially on SN and B&D, plus on how to use Mumsnet - to supplement the 'welcome: this is Mumsnet' email of course.
I think FAQs would practically write themselves as a result of a request by Justine et al.