[Hellmark]

Barnes & Noble have their own DRM, but they use it in tandem with the Adobe ADEPT DRM, in the Adobe renderer. It was supposed to be worked in, so other readers besides the nook would get it, but as far as I know that has yet to happen.

Kobo only supports Adobe ADEPT when it comes to DRM. This is the same as Sony, Aztak, BeBook, etc. With the exception of nook and Apple devices, if it supports DRM'ed ePub, then it supports ADEPT.

Kindle only supports Amazon's proprietary DRM, and they restrict the usage of their DRM. Any device that supports Mobi DRM, cannot support any other DRM without legal ramifications. Due to these restrictions, and others, only the Kindle supports it.

Your older ebooks from your Sony devices are very unlikely to be used on your kindle, ever (there have been rumors, but they're just that, and even if they come true it will not be any time soon). The books from your Kindle, are also unlikely to ever be used on anything but a kindle. Blame Amazon on that one.

iBooks is restricted to iPod touch, iPhone, and iPad devices. iBooks uses epub, BUT it has yet another proprietary DRM. No one outside of Apple is able to use it.

Most companies don't talk about DRM much. Some people are too technologically ignorant to understand what it is, and it upsets others, so they try to gloss over it. For instance you can buy DRM free books from Amazon, if in the description it says it allows for unlimited simultaneous usage. Many other places don't show anything at all to give you a hint.

Smashwords, O'Reilly, and I believe Baen all sell DRM free epubs. However, becareful, because books from those companies sold elsewhere may have DRM added by the retailer. This is extremely common on Smashwords books, where even the free books get DRM slapped on by third party retailers. If you're interested in something from them, it is best to go directly to the source.

As far as stripping the DRM, that is always an option, but be warned, you will not get any support here for that. It is illegal in many areas (such as the US, due to the Digital Millenium Copyright Act [DMCA]), and due to that, it is a forbidden topic.