[oppitronic]

I'll quote dpreview here:

The base sensitivity of the sensor used in the D5000 is ISO200. There is also a Lo 1.0 mode that attempts to mimic ISO 100 but it's effectively just ISO 200 over-exposed by a stop. The result is that the sensor becomes saturated and clips to white quite easily, limiting that mode's dynamic range. For most applications, you'd be better off buying a neutral density filter if you need slower shutter speeds than ISO 200 will allow.

(source: Nikon D5000 Review on dpreview.com http://www.dpreview.com/reviews/NikonD5000/18)



And my own experience matches that.

I only use Lo when I don't have my filter with me and want to take a wide open shot at bright sunlight and 1/4000s isn't fast enough.