There is not a dedicated WP for the Kindle however you can find a website that does allow text entry.
However the real issue isn't finding a text editor, it's the Kindle's refresh rate; the screen refresh rate is so slow that it really slows down your typing. Also when they refresh the screen it refreshes the whole screen not just the area your modifying.
The SONY does have note taking capabilities, they might have a better video controller. (I'm guessing here)