Hi
I think the new ones are an extension (rather than replacements - but for how long I can't say). I was just looking at the specs for all the Kindles and the Kindle Keyboard looks like it is the Kindle 3 we still all know and love.
Not sure about a firmware upgrade - some parts of the interface would be redundant (such as the onscreen keyboard) also the fact that the new Kindles have a touch screen as opposed to a keyboard means the whole interaction is different.
It doesn't make a lot of sense to me. I would like to get Silk as a update to my experimental browser on my Kindle but I actually don't browse that much anyway.
If you want to see the comparative specs you can scroll down to the bottom of this
page from Amazon.