Regardless of terminology, a massive attack on the virtual infrastructure is a different class of attack and requires a different class of defense. The term 'cyber Pearl Harbor' is ridiculous and disrespectful to those who were at the real Pearl Harbor. Also, DHS is probably the worst department to be in charge of, well, pretty much anything. The NSA would be a much better choice.
With that said, I think it's not too bad of an idea once you realize what the proper response is to a massive attack on the entire Internet. Technical support. Calling up (or visiting in person) millions of users and sysadmins and walking them through the process of securing their systems. A phone call might not do it, because you can't flash a badge over the phone, so (rightfully) nobody would trust you.
OTOH, targeted attacks such as "break into a wastewater treatment plant, use the SCADA controls to prematurely dump a tank of sewage into the clean water intake" mentioned by the AC, are yet another class of attack, and you don't need the extra manpower of a reserve force in order to deal with the technical fallout of such an action.
And, of course, if someone were to find something a new class of bug similar to SQL injection, wherein the only solution would be to update huge amounts of code all over the Internet, well, even a reserve force might not be enough to fix that kind of problem. Look at how long it took to resolve Y2K.
Personally, I would consider signing up for this type of "cyber-reserve", but I would hesitate doing so under the DHS. Also, I wouldn't sign up unless I knew ahead of time what was expected of me, and that there would be no bait and switch.