dr .dre used to use this whistle in hell of his beats back in the day like let me ride or, ain't nothing but a g thang, and gin and juice, and murder was the case they gave me? i'm trying to use that.
He used a Roland JV-1080 sound module. You might be able to find a some software patches similar online somewhere. On the new Reason 4.0 there are some patches in the Thor that sound pretty similar. Also you can make it yourself with a sine wave and a little tweaking. Good Luck!!!
It's a basic mono saw wave with some portamento(aka legato or slide) on it. Can be achieved on nearly any synth there is, just mess with the filter to change it from smooth (G Thing) to Harsh (Natural Born Killaz) however you like it.
Personally I like it best coming out of a minimoog, or an emulation of it. Check out a free one here http://home.no/gunnare/