Clinton,
I will respectfully but emphatically disagree. It would not be the first time that opposites have been read/written (especially in home marine keeping) but my understanding is that only a small layer of top sand harbors the desired arobic bacteria and that live rock provides far more of the desired surface area. Anything much below the first inch of sand starts harboring nitrates but no longer receives enough oxygen to be an effective bacteria culture zone to break down ammonia and nitrites. A very thick bed of sand (6" or better) will, over time, create poisonous (anarobic bacteria) that will desolve the nitrates but it is hard to maintain in anything but a dedicated tank and can be lethal if disturbed (difficult with many creatures, impossible with an octopus).
Having kept the old style saltwater tanks (and yes, Colin, I know you still believe in under gravels) off and on for numerous years with limited success or out right failure vs the success and variety with the seven marine tanks I now keep, I could never be convinced that even a fish tank should be set up without live rock as a primary filter.
Lastly, having been there and done that, cycling for a couple of weeks is a prescription for new tank syndrome, cycling for months is almost a guarantee against it.
Monty, we need a soap box or high horse smiley