Just a curious thing to note.... How would you handle situations where the target camera's orientation is a polar opposite of the current camera's orientation?
For example, camera A is rotated at 90 degrees while camera B is rotated at 270 degrees...
Seems lengthy to create a spline which keeps this transition "moving forward" since it would have to first choose a direction and then fulfil a 180 degree "S-spline" interpolation.
Just something to keep in mind
That being said, I do agree with Hodgman's suggestion over my own, but depending on what this is for, there may be certain scenarios where a more 'immediate' response to orientation could benefit the whole package.