A nice explanation for someone who has not looked into Arc (me).

[0-9a-zA-Z]{10} is what, 840 quadrillion combinations? Just out of curiosity, what happens if we get a 'fnid' collision?

I expect to be serving a few hundred trillion pages in a couple of years time :-p.

The article mentions that the fnid could be stored in a cookie. While this is true it definitely would not be desired, as doing so would make it impossible to browse the site with multiple windows,each on different pages.

And it wouldn't work if there were more than one form or link on a page. Or if the user clicked the back-button.