Ok, login isn't technically a verb... yet. Fact of the matter is, any language is an evolving form of communication (except maybe latin at this point). Along with words like signup, setup, etc, these words are nearly ubiquitous on the web, and widely accepted/understood.

...we can't let one little conjugation problem make thousands of programmers and technical writers look like fools

"login is not a verb" + "many websites use it as a navigation element" != "programmers look like fools"

I could use a simple door with an arrow image for logging in as well. That image is not a word at all. Yet it would still serve as an effective navigation element.

Also, who says that all websites intend for it to be a verb anyway? I have another navigation called "account". I do not want users to account the site. I simply want them to know that is the account page. Likewise, "login" signifies that is the link to the login page, for which this article granted that it may be used as a noun.

Ok, I've spent entirely too much time on this subject.