Dan,
By no means am I replacing my Phase back. In fact I'll be upgrading my P45+ to a P65+ when it becomes available in a month or so.
The Sony A900 system is what I'll be using in the DSLR catagory, but for what I do it does not replace the H2 and Phase back.
I should mention for the umpteenth time that what I do in terms of equipment is not at all typical of what others may do. I review gear for my site and magazine articles for a living. This means long term testing. As a consequence when something interests me I will buy it, using it for a time, and then sometimes sell it. The subsequent loss is a cost of doing business. Think Consumer Reports.
Most other sites get a camera sample for a few days or weeks, and then they're gone. Not all, but most. I sometimes do that as well, but not always. Things are never simply black and white.
On the other hand when gear satisfies me, I buy it and then keep it for a long time. This allows me, I believe, a unique perspective when reviewing. I often have many months or years of experience with a product and ten of thousands of frames.
Right now I've had my Hassy system for about 4 years and I'm on my third Phase back. I may switch to a Phase camera in the months ahead, but I'm not sure of that at the moment.
Michael