When you're building a web site, you'll often have a targeted experience for phones. They do many special characteristics after all: the browser is touch-driven instead of mouse-driven, the screen is much much smaller than a monitor, the user is likely to be on the move rather than sitting in front of a desk, the hardware is slower than that of a desktop computer, etc.
It's important then to consider how your site is performing, and to have tools to inspect how it's behaving in the browser. At datajs, of course we care a lot!
To help with this, Eric posted on Debugging Windows Phone 7 device traffic with Fiddler a few months back. If you have a Windows Phone 7 at hand and want to see how it's behaving, this is definitely worth reading.