There are several possible ways to create a headlamp that offers both spot and flood capabilities:
Tikka XP: A Near Miss?
The Petzl Tikka XP, which has only recently begun shipping, is an example of a product that incorporates such a diffuser into its design. Unfortunately, the output of this new headlamp, which is probably the EOS' closest direct competition, is unregulated, which means that light levels fade continuously as the batteries are drained. The EOS, on the other hand incorporates electronic "booster" circuitry that maintains fairly constant output even as battery voltage drops.
In addition, some early field reports suggest that the Tikka XP's sliding diffuser may, in fact, over-diffuse the beam, requiring the lamp to be set to a high power level in order to function well in camp. I have not yet experimented with this lamp, so can offer no personal observations, but will say that if a manufacturer were going to error on one side or the other, it would probably be preferable to under-diffuse the beam, since additional diffusion is easy to add-on (while the converse is not true).
Nonetheless, the Petzl diffuser design seems to have resonated with those observers who have commented thus far through online forums. It's also a design that I wouldn't be surprised to see Princeton Tec emulate in the next iteration of their product. So until the EOS II is available, a flip-diffuser equipped, original model EOS may be the best all-around headlamp for backpackers that current technology has to offer.
Some enterprising backpackers have suggested that diffusion can be added to the EOS simply by attaching a piece of tape of some sort over the lens. I made, in fact, exactly this recommendation through posts on a couple of outdoor forums earlier this year. As I've discovered since then, however, this method is not very convenient if one needs to switch frequently between diffused and non-diffused modes. Further, the tape that I think works best for this purpose (see main article) leaves a sticky reside on the front and sides of the lamp wherever contact is made.
So rather than attaching the tape directly to the headlamp, a better solution is to apply it to a door than can be flipped away from the lens when it's not needed.