Sony revised the PS2 slim quite a number of times -- but the changes were so minor (on the outside), that it was hard to tell the difference.
The same thing was true of the phat PS3 as well. External ports got removed, bays got sealed shut and removed, the guts changed... but most people think it was "the same".
I doubt the current PS3 will change form factor, unless there's a significant cost savings, and that likely wouldn't be until the chips were downsized to 32nm, or even 28nm, and the corresponding power supply was reduced in size, etc.
The last model of PS2 slim was the first PS2 slim to have the power supply built into the unit, although not many people took notice. Most people think the slim on the shelves today is pretty much the same as the one put on the shelves in 2004 -- but its pretty different. I have a 77001 (circa 2006) and a 90001 (circa 2009), and they are quite different, upon closer inspection, but you'd think they were just different colors, at a distance. likewise, the 77000 is different from the 79000 (lightest unit, by several hundred grams -- with no internal PSupply) and the 74000 (2nd model) and the 70000 (the original slim, I think). So there you have it. Sony, being a efficient hardware company, constantly revises their products.
But... I doubt there will be another "major" redesign of the PS3. It will, however, almost certainly change very slightly.
