I bought the 13" unibody MacBook one month prior to it's upgrade to become the new improved 13" unibody MacBook Pro (with unopenable battery compartment, extra 3 hours of battery life, more color gammut, SD slot, firewire 800 port, and a new 8Gb memory limit).
My MacBook says it is limited to 4GB of DDR3 1066mhz memory, in 2 SO-DIMMs. But that was written back when you couldn't get 2 4GB SO-DIMMs. Now that you can get them, the very similar MacBook Pro is shipping with 4GB standard, and can be upgraded to 8GB. I've asked (Apple reps), and I'm repeatedly told that my model cannot be upgraded to 8. When I ask for the reason they alway say: "Because that's the published limit at the time your mac was built." I find this unconvincing. If they said: "Because the memory controller in the chipset is limited to 4GB, despite being seemingly identical to the memory controller in the same chipset newest MacBook model," then I'd just take their word for it.
Has anyone tried it, or found any research as to whether two 4GB DDR3 1066mhz SO-DIMMs can be installed in the unibody MacBook without FireWire?