The main finding here is that web content needs to be simpler for smartphone users. Unsurprising really, given mobile screensize forces more zooming and scrolling which interferes with the process of scanning and reading the information on the page.
Cloze Test for Reading Comprehension - article and test demo by Jakob Nielsen
User comprehension scores on the Cloze test were 48% of the desktop level when using the iPhone-sized screen. That is, it's roughly twice as hard to understand complicated content when reading on the smaller screen.Nielsen goes on to explain why this is the case. In fact, he expects in the real world the problem to be greater. The study he comments on was conducted in a lab. In the real world we're using our mobiles on the move with all the extra distractions that brings.
I find all this particularly interesting as most conversations I hear about getting websites set for mobile consumption revolve around the technology. There is little or no interest in considering what needs to be done with the content and structure.
A site like the BBC is a great example of what to do editorially. They already produce news content which is concise and readable. But access the content online and you find that they have taken some clear editorial decisions on how much they present - you can't read everything you could via a PC. From the outside it looks like they prioritise the high demand content; the long neck, in Gerry McGovern-speak.
Mobile Content Is Twice as Difficult - article by Jakob Nielsen