Rafael Vázquez writes about evaluating the difficulty level of games on Gamasutra in the feature How Tough Is Your Game? Creating Difficulty Graphs:
They are graphical representations of how difficulty changes throughout the game. This is to say that they plot how challenge changes over time. There are two main types, time-based and distance-based. The first places the spikes in challenge according to the time spent played (taking away paused time and death); while the second places them depending on where the challenges appear (assuming a direct route from start to goal).
While the method seems to be targeted to combat-based titles (difficulty formulate uses to the number of enemies), the same idea could be extended to platformers by counting number of jumps (etc.) and multiplying that with a difficulty level?
Petri Lankoski 