If I create a big shere with a constans material in XSI and put for example a plane with a shere inside this spere, turn on GI for the light and the objects, then how come I dont see one of those soft round shadows GI usually causes? If I do the same in for example Lightwave it creates a nice shadow under the sphere. is this because LW has radiosity and not GI as XSI? Whats it the difference between GI and radiosity?
You are looking for is Final Gathering. Turn it on and then your outer sphere will emit light.
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Whats it the difference between GI and radiosity
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Basically they are different approaches to generate a light bouncing effect. Take a look at this link:
http://www.cgtalk.com/showthread.php?s=&am...al+illumination
Hope this helps
Salutations - Cheers
Bernard Lebel
Now I'm getting really confused... when did the army come into this? And I thought GIs were only ever sent on very special missions... well I be damned if everyone's got them! 
When you add GI to your scene, as soon as you hit Render you have a full squad of them knoking to your door. Very useful for collecting debts or intimidating rivals.
Bernard