This is only an issue if you're buying e-books from Amazon. There are plenty of wonderful texts available in the public domain, already formatted for the Kindle [0,1], and plenty of other options for public domain texts in other supported formats [2,3].

But regardless, the Kindle natively supports PDF, HTML, and plain text, and should you want to, Amazon provides a tool to convert those and other formats (including ePub) to the Kindle's proprietary mobipocket-derived format. They even provide Linux binaries [4].

Don't want to deal with conversions and manually managing your library? Just use Calibre [5]; it's open source.

And that's exactly why I placed my order: I'm not willing to accept Amazon's DRM, which will force me to read older works to get value out of the device. A goal I've had since reading a quote dubiously attributed to Einstein: "Somebody who only reads newspapers and at best books of contemporary authors looks to me like an extremely near-sighted person who scorns eyeglasses. He is completely dependent on the prejudices and fashions of his times, since he never gets to see or hear anything else."

[0]: http://manybooks.net/

[1]: http://www.feedbooks.com/

[2]: http://classics.mit.edu/

[3]: http://www.gutenberg.org/

[4]: http://www.amazon.com/gp/feature.html?ie=UTF8&docId=1000...

[5]: http://calibre-ebook.com/