A few days ago I came across Duokan, an open source firmware for the Amazon Kindle (2/3/DX). It's being developed by a group of Chinese developers and while it's still in beta, unfortunately, it's gotten good enough that you might want to try it.
BTW, Duokan doesn't replace the existing Kindle firmware, so you won't have to worry about breaking your Kindle.
Right now Wifi isn't working (for me), and I'm not sure they plan to hack the 3G connection. But you can read Epub, Mobi, PDF, and txt, and you can highlight and bookmark. I haven't tested it yet, but you also have the option of TTS (it's probably Chinese only at the moment). There's also a music player and what appears to be an RSS Reader.
I'm going to pass, myself. Yes, I'd love to be able to read Epub, but Duokan currently supports only full justification and I much prefer to have left justification only. Also, I'd really prefer to have Wifi. And to be perfectly honest, if I wanted an ereader that supported Epub I'd get an ereader that supported Epub.
You can download a copy of Duokan. Pay close attention to the instructions on how to switch the interface from Chinese to English.
Doukan via lesen.net