Unfortunately, no. The number of new posts is information kept only for forums, and there isn't really any easy way to make that information available for categories in the main category loop. Because calculating new posts is not such an easy task (there is a lot of calculation involved and multiple queries) it is only done for forums that will be shown; the information isn't even calculated for the forums of collapsed categories, much less the categories themselves.

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Jason Rhinelander
Gossamer Threads
jason@gossamer-threads.com