My first reaction to Nielsen’s article on the importance of designing for multiple viewports was that he’d misinterpreted the meaning of mobile first:
Some even recommend designing websites for mobile first, and then modifying the design for the desktop PC as an afterthought.
To me mobile first is simply a way of writing CSS that, ironically enough, overcomes a designer–centric approach to screen widths. Instead of placing a bunch of mobile–friendly styles in @media queries that only modern mobile phones can understand, it caters for older devices by putting the desktop CSS (two or more columns, for example) in the @media queries.
Nothing to do with adding desktop design as an afterthought.
But a Google search tends to back up Nielsen’s view of things. Luke Wroblewski’s article came top of my search for mobile first, and it goes way beyond how we should structure CSS. After all, mobile is ‘exploding’; by the end of this year smartphone sales will outstrip those of PCs.
Smartphone sales ≠ the death of the desktop
Although this may sound strange coming from someone who built a mobile first WordPress theme, not every site needs to be mobile first.
They're not looking at your site.
If smartphone sales are increasing that doesn’t mean hordes of people are visiting your site on their iPhone.
Context is everything. iPhone owners are viewing their Twitter streams, Facebook and Gowalla, but, as Nielsen points out, you’re still going to do certain things on a PC.
Check your analytics. I’m willing to bet that less than 5% of your blog’s visitors are using a mobile device. They’re more likely to be using IE6. The 95% should really come first.