You would only be able to pin one wall post to your profile. You can always just unpin a wall pot to pin a new one. I think the pin button should be next to the delete button of a wall post (any wall post, in your profile. Don't delete stuff by accident though :P). I feel like this could be a useful feature (e.g. You pin, "I'm going to be offline for a while" so it doesn't get covered by new posts or one very long post. You could pin anything, like, "I'm doing a shiny Natu giveaway"). There also has to be pinned, next to pinned wall posts.
I'm pretty sure not everyone's walls are dead so this won't be completely useless. I'm sure dead walls would like pins too :P.