To add something new to the discussion, I decided to check the low ISO ranges for various formats, possibly ignoring the marketing spec (using DxO sensitivity measuremetn instead):
FourThirds:
Olympus E-3: ISO 118
Panasonic G1: ISO 129
APS-C:
Pentax K20D: ISO 87
Pentax K-m: ISO 98
Nikon D300: ISO 145
./.
(yes, the K20D has a better SNR than D300 at lowest ISO, but not a better DR ...
for D300, SNR (11.72Bit) and DR (11.59Bit) are equal; not so for K20D, SNR (12.19Bit) and DR (10.61Bit) differ
-- the explaination lies in the non linear full SNR log log plots ... K20D is less linear and better at brighter luminances, D300 better at darker ones
)
./.
Nikon D90: ISO 145
Canon 50D: ISO 157
Canon 500D: ISO 92
Sony 380: ISO 87
FF:
Canon 1DsmkIII: ISO 73
Canon 5DmkII: ISO 73
(would have stellar dynamic range if not flattening out below ISO 800
)
Nikon D3X: ISO 78
Nikon D3: ISO 161
Nikon D700: ISO 162
Sony 850: ISO 101 (DxO mark out already!)
Sony 900: ISO 119
Conclusion:
Currently, the low ISO boundary is more vendor-specific than dependent on APS-C vs. FF.
Pentax, Canon, and Sony seem to offer true ISO 100 where Nikon does it only for the D3X.
The low ISO boundary is determined by two factors: "full well capacity" (a CMOS process parameter -- the better the
lower the low ISO boundary) and "quantum efficiency" (another CMOS/microlens process parameter -- the better the
higher the low ISO boundary).
If I think about it again, the maximum number of possible photons (before overexposure) hitting a square millimeter should be independent of sensor size. Therefore, the low ISO limit should be independent as well, giving the larger sensor a possible advantage for dynamic range (allowing longer exposure at the same physical aperture or a wider physical aperture (same f-stop), so that more photons overall can be collected before the sensor is "full"). This is confirmed by looking at some P&S: Nikon P6000 (ISO 94, marketing: 64) and Leica/Pansonic LX3 (ISO 53, marketing: 80).
BTW, a curiosity are the Hasselblads which stay at ISO 47 whatever ISO setting you use