I've been reading several books on digital photography; some have mentioned the need for at least an f2.8 capability for low light, on flash photography. Does this mean all f2.8 capable cameras have theoretically the same low light ability?

I can find pocket, compact, prosumer, and DSLR cameras that all seem to have the same capability of taking a pic using a 35mm focal length at f2.8 (ignoring the use of flash for now). Does this mean that the amount of light in each camera reaching the sensor is identical?
(Theoretically making them all very similar in their ability to capture a picture in low light, quality issues aside?).

What function does all that big glass serve on the prosumer and DSLR cameras? Is it just for reducing aberrations, or is it also serving to capture more precious photons?

When all parameters are the same, the exposures will be the same. The larger lenses allow larger apertures to be used. So an f/1.4 55mm lens will have its front glass element having a diameter about twice as large as an f/2.8 55mm lens, and at maximum apertures, will capture 4 times the amount of light. If you look at the lenses of cameras that have relatively tiny lenses, don't be surprised if they're rated at f/5.6 or f/8.0. The really large ones will have maximum apertures of f/1.4 and f/1.2. I recall a really old, huge Canon lens for one of their rangefinder cameras that had an aperture something like f/0.95, and there was an f/1.2 58mm lens for the Nikon F. Not the sharpest lenses, but really good for taking pictures in minimal light, as long as you didn't mind lugging around large hunks of glass.