My first thought is that this can trivially be spoofed by installing a keylogger with playback functionality, but at that point a password wouldn't save you, either.
If you got through the problem of people's keystroke speed varying with local factors, you'd wind-up with a situation where not only is your "password" the same on every site, even sites you visited without logging into could "sniff" your "password".
0-factor "identification"!
Checking long posts on news sites or blogs just to make sure it is the writer would be interesting, but boring and not worth it.
http://www.cs.berkeley.edu/~tygar/keyboard.htm
"We examine the problem of keyboard acoustic emanations.
We present a novel attack taking as input a 10-minute sound recording of a user
typing English text using a keyboard, and then recover- ing up to 96% of typed
characters. There is no need for a labeled training recording. Moreover the
recognizer bootstrapped this way can even recognize random text such as
passwords: In our experi- ments, 90% of 5-character random passwords using only
letters can be generated in fewer than 20 attempts by an adversary; 80% of 10-
character passwords can be generated in fewer than 75 attempts.Now, visit it from a different browser or computer and make another comment - it would log you in as you again, somehow.
Errors would likely make it unpractical, but it'd be an amusing demo for the unaware.