No problem for me in my 06 HCH. The time that this condition occurs is at low speeds, say 25 miles per hour. Here is an example:

1. I accelerate up to coasting speed (25 miles per hour) on a flat surface, or even better on a slight downhill grade.
2. Take my foot off the gas pedal and wait for the mpg indicator go to 100% while coasting. (If you find that your car does not coast well, get more air in your tires)
3. I now press lightly on the pedal to maintain my speed or to accelerate slightly. (You should observe the assist bars at that time, while maintaining 100% mpg)

I am under the impression that this condition also occurs going down a large hill, but the assist bars are not shown because it is displaying that the battery is being charged.

Honestly, this condition doesn't occur all that often in my driving and doesn't have that great an impact on my ability to get good gas mileage. Things such as good tire pressure, conserving momentum, and a non-agressive driving style have a lot more to do with it.

Another clarification on this topic is to mention that "all-electric mode" in the case of the 06 HCH means that the gasoline engine is in idle while the electric assist is providing the acceleration. I mention this because a friend of mine assumed that the engine should shut off when in this "mode", which it does not.

Hope that helps.