Not entirely, if they join and post 50 messages in the first week and then don't post again for 30 days..... I guess you could set it to 51 but not only is that a weird number but "Hey, we see you haven't posted lately" when they just joined and haven't posted yet wouldn't work.
Some combinations of conditionals would be useful as well, e.g. has logged in within last 5 days but hasn't posted within last 30 days. These could be used quite nicely to encourage more posting from lurkers.
I think lurkers will always be lurkers, a notice won't make them post.
I would recommend giving something for people with x number of posts or trophy points, e.g. resources to download or other content that is not just member only, but only for members with x posts, or y frequency of posts (not sure if that's possible, but there must be an addon)
Above all keep the forum active, there's nothing like good content to get people joining in and sometimes the forum owner must be pro-active in getting the discussion started and flowing.
There is another reason for wanting User has not posted a [forum] message for at least X days: that some admins might want - usergroup promotions.
I think it would be great to be able to reward more active users and this is one way of doing it. If they are active and have posted in the last X days, they get promoted and get some extra privileges. If they don't then they don't get these.
I'd like to see a slight variation on this to:
User has posted X messages in the last Y days
This could be used in the way that @Clickfinity wants (he'd set X to 0 and Y to the number of days). I could also use it but rather than have someone who comes back, makes a post to get promoted and then disappears again, I could set it to say User has posted 10 messages in the last 30 days. That way I can more accurately award the more active members of the community with a user upgrade.