If you've already started saving stacks of gold for the next tier of riding in Northrend, you might as well go find something better to spend it on, because Bornakk has confirmed that Blizzard doesn't plan to extend the skill any further in the next expansion.

The current highest skill is 300, which allows players to ride epic flying mounts. This skill costs 5000g wow gold, and to my knowledge is the biggest single pay-out in the game to date. Speculation began last night, about the cost of the next riding level. Bornakk was quick to stomp on the discussion though:

Blizzard Quote: (Source)
We don't currently have plans to add another tier of riding skill to the game in Wrath of the Lich King.

Sorry, no flying bears planned.

Well, there will be things that cost money in Wrath of the Lich King, we have no plans to remove currencies.

Note: This has nothing to do with the purchase and/or process of allowing your flying mount to fly in Northrend.

So it won't be a higher skill level to fly in Northrend, but it may be a quest, an item, a rep thing, or a combination of all those things. It also may cost a lot (more time, more wow gold, and frustration...).