360 slim is coming because MS will be cutting cost just like Sony. The slim is not a response to Sony Slim but instead a tool for MS to reduce the cost of the 360, sell at a attractive price and still be able to bundle Natal and make a profit. A new form factor can also have increase in sales but that's a side effect.
The difference with MS doing a slim model and Sony is that Sony had to throw away all of their profit while MS is still in a nice position to make money on their revision. Forget that MS has stated that they see this Gen lasting a lot longer than the last. You can say that this is MS new console without having the steep development cost








