He has taken the "no fixed keyboard" argument seriously. He wrote about it a number of times. He came to the conclusion that (when implemented well) there is not much of a difference, except user preference.
It just depends on your definition of "taking it seriously". For example in that article Gruber says
Most people can thumb-type just as well, if not better, on an iPhone as they could on a BlackBerry.
I prefer the iPhone to a Blackberry overall, but the argument that there is no advantage to typing on a Blackberry is just not true. Here's a better analysis:
That's not an "analysis". That's an opinion just like Gruber's - who, incidentally, has much more in-depth articles regarding thumb-typing than this one, which treats it as an incidental issue.
Most people can thumb-type just as well, if not better, on an iPhone as they could on a BlackBerry.
I prefer the iPhone to a Blackberry overall, but the argument that there is no advantage to typing on a Blackberry is just not true. Here's a better analysis:
http://msgnet.org/2008/08/blackberry-vs-iphone-simplified/