The camera's base ISO is 200, with ISO 100 being an extended speed. On most cameras the use of lower than native ISO is usually not a worthwhile exercise since it simply crushes dynamic range and contrast. But that's not the case with the A900. What the use of ISO 100 appears to do is reduce headroom at the top end of the exposure scale, but opens it up about a half stop in the shadows. It also lowers the noise (which is already very low at 200) from a Noise Ninja reading of 11 down to 9. According to DxOMark the noise reduction is almost 1db, from a SNR of 35.2 at ISO 200 to 36.3 at ISO 100, corroborating what is seen in Noise Ninja.
Based on an extensive series of tests, I am now shooting with the A900 at ISO 100 when I can, which means when working on a tripod and when shadows are more important than highlights.
BUT: But, be aware that by reducing the ISO below the camera's native 200 you are sacrificing one of the A900's great image quality assets, and that's its very broad highlight "shoulder". To my eye this gives the camera a very distinctive advantage in that highlights seem more film-like, with less of a tendency to clip, and to show more nuanced tonal and colour separation in lighter tones. For many current and prospective users, especially those with a sophisticated appreciation for the nuances of image quality, this highlight latitude may provide the camera's biggest appeal, so only switch to ISO 100 in situations where slightly lower noise and more shadow range are what's desired.