I don't think looking at time spans of the previous PS slim releases is the best way to analyse the position of the PS3 slim as this time it came about out of neccessity rather than elongation of product life-span. I wouldn't put it past Sony to have plans already in place to elongate the lifespan of PS3 as a product. They keep touting this 10yr plan so it wouldn't suprise me if they have at least some secret plans ready for execution.
Also, whilst Natal may be getting all the attention, Sony's dildo... ahem... "Wand" will also be releasing, and probably for cheaper than Natal (Natal looks to incoporate more complicated tech than the Wand). I also don't see MS releasing a revised slim version of 360. Partly because their strength isn't in hardware, but software and also because they'd need to design the it around some of the periphals (mainly the HDD).








