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:
http://daringfireball.net/2009/07/mobile_phone_keyboards