I think the biggest leap for Kindle was the Kindle 3 (aka "Keyboard Kindle"). If they support that, they should be fine. As far as I understand, the newer Kindles (touch, non-keyboard) use the same software and processor as the Kindle 3, so there is nothing preventing them from supporting it.

Besides, Kindle can render full blown websites and PDFs in its "experimental" browser, so some formatting enhancements in the Mobi parser should not be too much of a problem.

I hope.

Good thing Amazon is not Apple.