If it's a placeholder, it will need to be like this, otherwise you'll get the date of the page with the aggregator tag (e.g., getResources) for every doc:
[[+publishedon:strtotime:date=`%B %e, %Y`]]
Note that the date modifier calls strftime() not date(). The format tokens are not the same. The ones you want are here:
http://us1.php.net//manual/en/function.strftime.php
Also, you may already know this, but pub_date and publishedon are not the same pub_date contains a future publication date and is usually cleared when the doc is published. The publishedon field holds the date that the doc last changed from unpublished to published (or the date it was created if it started out published).
Whether it works or not will depend on the parsing order, so you'll have to try it to see.