Something better than the Vita should be fairly easy for Nintendo to manage.
The Vita is basically an iPad 3 chip, the newer iPads blow that out of the water, and Nintendo still is a year out from their product launch.
I think they will push their handheld to be more powerful than the usual "super outdated" philosophy of past handhelds for a couple of reasons.
For one it will allow for easier porting -- the whole point of a unified platform is to allow for very easy porting, if one system is radically more powerful than the other the whole setup falls apart.
The iPad generally tends to be more powerful than the iPhone, but usually not by a ton, so apps run seamlessly on both platforms with no problem whatsoever.
The other reason is to differentiate their handhelds from lower end tablets, giving it more grunt under the hood is one way of accomplishing that.
Next gen will be the first cycle that Nintendo will also need their handheld to be able to play all their big-gun IPs like Metroid, Zelda, 3D Mario, etc. Not just hand-me-down/scaled down versions of the console games, it will have to be able to run the same games reasonably well. So the handheld becoming more tech driven makes sense.
Nintendo didn't need the Game Boy to run Super Mario Bros. 3 or the GBA to run Zelda: OoT or for the DS to run Mario Galaxy. That dynamic will change, the handheld will in effect become the new lead platform for Nintendo's dev teams.
