If it was in, they probably would have said so. I could be wrong, and I'm not going to get my hopes up.
How will they balance it though? Some archtype playstyles may benefit more from mounted combat than others, you'd want to balance that so your stealthy Bosmer has just as much of a chance to kill a troll on horseback as a Breton Mage, or a Nord Warrior.
What's so bad about that? If being on a mount is an advantage for your character, you use it. And if it's not...you either dismount when you have to fight, or you don't use horses at all. Truth be told, I wouldn't imagine someone whose lifeblood is stealth to get around the wilderness on a big, noisy animal which looks quite tasty to most of the fauna. Especially to the "invading" species.
You apparently have no clue how mounted combat works in real life. Either you A: are fighting an enemy ALSO on a horse B: are using a bow, or C: fighting with a javelin. Last time I checked, we don't have Javelins in Skyrim.
You going to swing your short sword while on a horse at a 3 foot tall boar bro? You wont do much damage considering you wouldn't hit it.
I play a Warrior. I always play a Warrior. I use two handed axes. Do you realize how absurd it sounds for me to be riding a horse, a huge 240 pound Nord Warrior in full plate armor swinging around a 80 pound waraxe at a boar at my feet?
I think you should be more worried about using the 80-pound waraxe
at all before considering using it on horseback.
A expensive (in terms of develoment time) solution will be addding limitation on certain zones that dont allow free horse movement.
Expensive? Doubtful, considering that's probably already in, to a degree. Like Oblivion, it's unlikely we'll be able to bring horses into interiors and walled off cities.