#0 - Sept. 17, 2010, 3:12 p.m.
I believe this is because of the (awesome) new feature that converts flying mounts to ground mounts so they can be used in Old Azeroth and on Dalarans ground. Since Flight Form hasn't been given a "grounded" version the mechanic that makes it convert when you go to Old Azeroth doesn't effect it. I'd suggest you force flight form to cancel before you can use portals if this feature is intended to make it to live.
I figured it's only right I report this pretty major bug here but I'll admit I was a bit naughty and used this opportunity to do a free-roam fly-over of all my favourite old world zones and levelling spots with graphics on full and all sound and music on full volume. Just a really good chance to say goodbye to the world I've spent so much time in, I won't deny it nearly bought a tear to my eye :)