Looks like Raspberry Pi will became very popular and it's a big chance for promoting ReactOS(if there will be an effort to port it to ARMv6Z). I'm not very familiar with multiplatform coding so correct me if i'm wrong: the main idea in porting ReactOS to ARM is replacing ASM code into C in parts where it's possible and use ARM ASM where it's not possible. So switching between different ARM versions should be mainly just matter of ASM syntax. And this is not so hard (afaik) as converting ASM to C.
Nope, there is much more work involved. There are big differences between x86 and ARM architectures so the port needs some rewrites, you can give a look to the ARM branch. Sadly seems to not be so easy.