I wouldn't worry too much about being locked into the Kindle ecosphere if you're willing to strip DRM. The vast majority of the books at Amazon are .azw which is Mobipocket DRM. They can be stripped of DRM so you can archive them. That way if you eventually get another device, you can still read them. Topaz format books (.tpz) can't be stripped but luckily there aren't many of them. I can't give the best description of the format. Other folks here can. Essentially it's a scan of a book. There's been some amount of OCR because it's searchable and dictionary lookup works. It's a quick way to get back catalog books into digital form with little effort.
By the same token, most other ebook formats can be stripped of DRM and converted so if you decide you want to buy elsewhere, you can. Sony's .lrx can't but that's going away. They, like many others, are moving to ePub with Adobe DRM which is strippable.
As for modding, you may also want to look for the thread in this forum on screensavers for Kindle 2. You can load your own images if you want.
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