I saw an interview one night on Charlie Rose with the CEO of Amazon (his name escapes me at the moment), and he said that a lot of the cost was the ability to download books through the cell networks without an actual contract with any service. The cell phone makers can subsidize that cost by locking people into contracts when they buy a phone (dropping the phone cost), but with Kindle, you're paying for all of that up front. Makes sense, when you think about it (not that I did a very good job explaining it - he was very eloquent though).
I do think it would be nice if those of us who didn't need "instant access" all the time could buy the device, and just download books through a web site using a simple wi-fi antenna (or download to our PC, then upload to the Kindle). Something that doesn't require use of the cell networks. That would lower the cost, and I already do that with my PDA, so I'm used to the process...