Horde:
I've seen posts about it being possible to reach the alliance wintergrasp portal and use it - when alliance is in control and your own portal isn't up. And I'll explain the easiest way to get there for horde characters.
Here I go:
Enter The Violet Citadel and take the portal named Portal to The Purple Parlor. (Up the stairs and then to the right)
When you have teleported up, run straight forward, out of the room and onto the balcony. %26#8216;Aim' yourself so that you see the alliance part of the city, and simply mount up with your flying mount. (Yes its possible to mount up here!) Now just fly into The Silver Enclave and use the portal*! - Tada, you are now teleported to the horde camp in wintergrasp!
Using this way you will not even get the "restricted flight area"-debuff and you will not risk %26#8216;loosing time' from having to retry it!
* Do not land your mount, because if you do, you will get teleported out like if you were running on foot.
Note: If you're a paladin, you can of course just bubble up and run into the portal without the guards beeing able to portal you out.
Alliance:
Simply go to Kraus's Landing, mount up, and fly straight to the wintergrasp porta*l. Ignore the "Restricted area"-debuff and hope you make it! Some scouting on the map may be needed. I'm sorry I can't write it better for alliance, but I have never tried as alliance. The idea here seems very possible and easy tho!
* Start with flying along the utter wall of the building just north of Krasu's Landing. Turn left immediatly when the wall %26#8216;turns' and then right as soon as possible - then go straight forward into the doorway and use the portal.
by Mollymus