Jakob Nielsen's on having a mobile site vs. a full site:
Good mobile user experience requires a different design than what's needed to satisfy desktop users. Two designs, two sites, and cross-linking to make it all work.
He basically says you need 2 completely different sites, and that for the mobile site, you should "cut features, to eliminate things that are not core to the mobile use case; cut content, to reduce word count and defer secondary information to secondary pages; and enlarge interface elements, to accommodate the "fat finger" problem."
In a way, this makes sense. Navigating a website designed for desktop use on a mobile phone is frustrating at best and downright impossible at worst.
However, I don't agree with cutting down features and content. Just because you're on a mobile doesn't always mean you're just after a quick fix or don't need all the information. I'd argue that many people today will use their smartphone to browse the web just because it's at arms reach and they can't be bothered to get up and open their laptop. This doesn't mean they need less features and content.
The challenge, I think, is a design and usability one. And it's not an easy one.