I searched for a few keywords before posting, if this has been posted before... oh well:
This has been tried and tested alliance side, not sure if it works for horde. It's good for low levels who need to get to the other continent (for UD or alliance) or if you want to buy rep gear from WSG w/o all the travel.
Queue up from any of the three cities, once in queue right click on the button that shows up by your mini map, then click change instance. Then requeue from that screen. When your match is over, you will exit from the ashenvale queueing location (or barrens respectively for horde, unconfirmed).
If somebody could comfirm this for horde and let us know, that'd be great.