Mobile use is one of the biggest challenges now facing many websites. Its also important for some intranets, particularly in companies with many traveling employees.
The challenge has been around since the PalmPilot + application specific tools.
I suspect the success rate for doing “phone” tasks or “address book” tasks is much higher than the “web” tasks.
A custom application (which is all that a “Mobile Site” is, really) is going to provide a better experience.
Simple example? Google Maps – can’t use it on a “mobile” browser. The custom application for my phone works quite well.