[moomal]

I did this on 4.2 in products_t.tpl (before reading this post) using the following code:

{if $smarty.now-$product.add_date lt 2629743}NEW!{/if}

We're using styled text, but obviously you could replace NEW! with an image if that suits your needs better.

The 2629743 is the number of seconds in a month, as we wanted anything added in the last month to be marked as new. If you want a different time period, you just need to figure out how many seconds long it is and then use it in place of 2629743.