Looks like I opened a can of worms with my comment about being locked into buying through Amazon.
It was not clear to me how content was loaded onto a Kindle. Every place I looked it just talked about transferring via WiFi or 3G from your Amazon account. As I said I like to have complete control over my device and my library - did not like the idea of having to hand this over to a third party.
Also the vast majority of my books are in ePub format. I know you can convert them using Calibre but to be honest I really was not interested in having to do that just to be able to read them on a Kindle.
Anyway at least I now know that if the worse case scenario does arise and Sony walks away from eReaders, and my 650 packs up, there is an alternative, even if it's not as good as the Sony.