Thanks for all your input so far! Let me recap where we stand right now.
In general, most of us agree that it's a good idea to sort by vendor and to make each vendor a main category of our community (rather than placing devices under "E-Book Devices").
It was suggested that we keep vendor-independence a main feature of MobileRead; that is, we shouldn't try to separate our members if it can be avoided. I think this is very important so let's keep this in mind while we are having the reorganization. Important: Let's only separate where it makes absolute sense.
One difficult question we have to ask ourselves is as to when a vendor would qualify for its own main category. Sony, Amazon, iRex, and Bookeen are obvious cases. Not as obvious is eBookwise, which, although no longer actively developed, still enjoys much popularity among our members. Then there are vendors like Plastic Logic, from which we expect great things to come, but that's all in the future. Someone suggested that we should look at the level of activity as depicted by the number of posts or threads, and judge from there. Others argue that activity for a particular device may increase just by giving it more visible attention (by putting it under its own main category).
It was suggested to split the existing Sony PRS 505/500 forum into two forums and place them both under the Sony vendor main category. I personally have my doubts about the usefulness of this, given that the 500 has been officially superseded and that the extra amount of work it takes to sort them out may just as well be used for something that is more future-oriented, but I understand why a separation could still make sense (after all, there are plenty of 500 owners out there).
Similarly, it was suggested to split the software and development sections for the iRex iLiad and the iRex Digital Reader, since both are completely independent and unrelated devices. Here, too, a splitting of the existing software and development section would require some extra work.
Another important topic is how to deal with OEM devices, where software is the distinguishing factor. I was told that Jinke is the main company behind those devices, so it may make sense to make Jinke as a vendor a main category of our forum and to put its OEM devices underneath it. As for the OEM devices, we may either use a single forum (with thread prefixes for the different vendors), or use individual forums, in particular if a device is to receive more attention.