The 9300m is not the same as the 9400m shipping with the new macs it seems? I guess what I really meant was the "Geforce 9400m G" the one with the chipset and graphic chip combined. Please let me know if they are the same..
Wondering if a machine using it will have better battery life / temperature / weight.
A 9300m and a 9400m are both just chips, and they both perform similarly. When paired with a second, smaller GPU in hybrid SLI (or similar technology involving switching GPUs) you can improve battery life by only using the high-power GPU when you need it. The 9400 is not an enthusiast GPU, though, and it's usually paired with more powerful graphics cards (like the 9800) and used as the low power one. I don't know where you read that the macs are using dual GPUs, but they aren't (as far as I can tell, at least. It's not like they have a need for one...). Any 9300m will perform almost identically to any 9400m.
Where did you read that the 9400 was paired with a smaller GPU for the hybridization? The 9400/9300 series is nvidia's smallest GPU, I don't know what else they'd be paired with. (unless you can use an Intel chip with one, which I'm not aware of being possible)
I haven't heard anything about dramatically reduced battery lives or anything of the sort..and it is still a fairly powerful GPU. Taking down your screen brightness is a better way of saving juice. An intel chip will give you better battery life, period. They just aren't powerful by any means.
Sony Vaio Z has an Intel GPU paired with a Geforce 9300m. It can toggle between them without rebooting, so you get decent 3D when you need it, and longer battery life when you don't. The machine is pretty small too, at 3.5 pounds.