Actually the N1 has 512mb of usable RAM. Some is used for FB, GPU, AMSS as for every phone, and we can explect that to be about 100 to 150mb.
The Rest is totally available to linux. The .29 kernel is simply unable to address anything over 256mb. That means that it can only see 256mb curently. About 25mb of this is used for things like the framebuffer and shared memory. The majority of the RAM used by the other resources like the AMSS (the 100 to 125mb) doesnt need to be addressed by the kernel so can be used above 256mb.
With the .32 kernel which will be released soon, the kernel is able to address ALL 512mb. That means we will have 512mb - 100 to 150mb available to android.
The ram isnt HIDDEN and there's no hack or crack to reutilise it as such. Cyanogen just patches in the himem addressing that the new kernel will use.
As for different main boards. The main board can be identical for both 256 and 512mb boards. They simply drop in a different SRAM chip and compile the kernel differently. This isnt difficult or costly for them to do.
Its plausible that the Bravo could have 256mb RAM, although unlikely. By the time its released 256mb RAM will be 'average' and not future proof for new android builds.
However they could do it as a marketing ploy or to maximize profits. Most people wouldnt know the different. Look at the 32A, 32B mess with the magic.
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