Look back at the very first screenshots brought out; there's trading posts in them. The one in the spoiler, for example, has a couple that the Celts have done.
On reflection, perhaps all that's happened is that they have been moved to be introduced around Medieval times. Most (if not all) of the many screenshots we've had are from people doing a 90-turn trial Firaxis must have sent to games journalists and they'll have been more interested at looking at the new stuff than building trading posts.
Spoiler :

On reflection, perhaps all that's happened is that they have been moved to be introduced around Medieval times. Most (if not all) of the many screenshots we've had are from people doing a 90-turn trial Firaxis must have sent to games journalists and they'll have been more interested at looking at the new stuff than building trading posts.