At the moment not stock device is truly open, but on the same coin no device is truly closed. All devices have some sort of hack to personalize the device, though some are more customizable than others. The irony here is that most eInk devices are built on top of Linux or Android(which is built on top of Linux).
So aside from taking extreme measures like wallcraft suggested, such as buying a device with the intent to install OI then most of the eInk Devices are on the same playing field.
If your concern for 'openess' is more along the line of book format, then you really want to steer clear of DRM. There are stores like Oriely(tech), Bean (a SF/Fantasy store), Smashword, and a few others, that sell DRM free books. These store also tend to sell the major formats like Mobi, ePUB, and PDF which cover the majority of the readers out there. There are also a multitude of sites that offer free books that are either in public domain or fall under common license. A great site is Feedbooks.com
The 1984 incident really had a terrible effect on Amazon and they have promised to never do that again. However this kind of remote behavior is not unique to amazon. Google has recently removed applications that it considered 'malice' from remote devices(smart phones).
I've own the SONY PRS-505 and the Kindle 2 and both offer a great experience, as a person who also grew up on the "open" foundation, I can say these devices are pretty open. So long as you are willing to liberate your DRM books.
If you are open to smartphones there are a great amount of devices coming out that offer 3.7", 4.3", and 5". Such as the the HTC Incredible, Evo/Droid X, and Dell. With the great app support that exist for the Android market you can read pretty much any format and any DRM on these devices.