I'll go next.
I have recently figured out how to map the unused memory on 2GB machines into the address space. You may have noticed me ask people to test something called MPMapper a while back. I had figured out how to do it, but a bug in the NanoKernel prevented it from working (It crashes 100% of the time if you run it on an unpatched ROM). I discovered and fixed this bug around a week ago, so we can now map in all the memory (if you have a Mac OS ROM that has the bug fixed. It hasn't been distributed yet).
By itself, this isn't enough to break the 1.5GB 1.625GB (thanks ELN!) barrier. The memory is part if the address space, but it is in its own separate chunk off in the middle of nowhere. You can access it, but the Memory Manager and Process Manager can't use it. I suppose you could tell the Memory Manager to create a private heap zone there, but that would only be useful for the one application that did so.
This is a step of the way there, but we would need to patch the Memory Manager, the Process Manager, and maybe the Virtual Memory Manager to make it usable for the entire system.