@BK I defer to Daniel's expertise on Canon's A/D converters (ADCs). He stated that for lower pre-amp gains, Canon's ADCs are terribly noise and non-linear. So by his estimate ISO 100 and 200 are basically the same. In a perfect world we could have ISO 10 for really sunny days at f/1.2. However, there's a minimum amount of amplification necessary to actually get the signal moving - hence no super-low ISO. Some medium format digital sensors do offer sub-100 ISO values. This makes sense as their sensor areas are larger as are their pixels (generally) so there's more photons to work with at the lowest levels of amplification.

So, what do we do to get more signal from fewer photons? That's the $1M question. You've heard of backside illuminated sensors? That's one way. Improving the gain/noise in the pre-amps is another. Reducing the spurious noise sources helps (active cooling above), better energy-level matching for the semiconductor junctions to photon energy... There's a bunch of technologies at improving CCDs, CMOS, etc. Alas, the really advanced stuff is not destined for SLR for many, many years after it's available in scientific-grade sensors for quite a while. True, the market is huge for pro / pro-sumer / enthusiast DSLRs but the margins are thin. The scientific instruments have huge margins but very thin sales. But no self-respecting researcher is going to pay a 10x markup (or more) for a camera that's no better than what he can buy from his local box store. So the sci-grade stuff has to be that much better.