Not exactly mail trading, but there's a lot of selling and co-op orders in the arcade and unusual consoles "scenes", and therefore a bit more of the old feeling/give and take. Also comics and unusual computers, although eBay etc has minimized that.
Everywhere where there is homebrew, young talent that can be molded exists (insert evil laughter and rubbing of hands, oh and bushy eyebrows). I'm not super sure, but I think there's at least a little in iPhone/Xbox (not 360), maybe NDS and Pandora too. PS3 - not sure. Dunno if anyone but seasoned gamedevs even know/care about the demoscene. Wii is possible, since the docs seem to be there.
Windows - well, there's not much new hardware to explore there - maybe some new nice shader or some trick distributing code over the GPU. I mean, the new hardware works like the old hardware, hardware differs a little from each machine and so on. There's Larrabee, but bending that from inside the current framework will be hard and incompatible.
The conclusion for most of us is that to a) do something impressive, you need a fixed and limited platform (at least not "GHz") and b) make a new type of demo, you need a platform that is distinct from the others (ie., "not plug in some c code in a wrapper and wham! it's in the pixelbuffer"). The 4K etc categories are an attempt at the former.
Even if it's neither impressive nor new, bending a chipset is the only thing worthwhile for me. And so when looking on the choices of platforms, Amiga looks attractive again to those who want to at least have fun while doing all that work
Of the modern CPUs, ARM is the most attractive CPU to code in asm. Most of those sit in portable [gaming] devices. As long as the hw docs are there, they could be attractive.
The boundaries are gone but in its place is internet in our kitchens stealing everyone's cycles.