i only heard about this "new mattress for each baby" thing from DH who is the more paranoid one about everything. i think he read in baby bargains but i couldn't find it.
i did an internet search and apparently this was discussed before, six years ago:
http://www.windsorpeak.com/vbulletin...d.php?t=134970
also apparently in a UK study, reusing mattresses can increase risk of SIDS, due to bacteria that has been sitting around for a while
http://www.gurgle.com/articles/Home_...cond_hand.aspx
the whole outgassing of chemicals by new mattresses was apparently a myth, as SIDS studies showed there was no link between increased SIDS risk and chemicals in traditional (non-organic) mattresses (p. 100 on Baby Bargains)
anyway, i didn't that it was that big of a concern but considering a new mattress is only $80-100 i figured might as well get a new one when we have DC #2