I think Avatar covered it pretty well there. If you check the board often, it's certainly easier to have newest at top - who wants to scroll past the same old stuff every time to get to the new posts (unless you want to carefully click on that little orange box that's about 3x3 pixels).
Weblogs always have newest entries at the top, newspapers don't put the old news on the front page, tv news is the same - newest stuff first, so why not forums? Most often visitors that read the forums are regulars, so they don't need to scan through all the earlier posts, but I think why forums started out oldest first is because intuitively we read from top to bottom (at least in English) and it was kind of logical the entries be ordered that way too, but it's not really as practical considering the frequency of visits.. if that makes any sense..