It would be nice to be able to scroll just dragging the content, like on the n810 or on the iPhone. Probably this feature is useful only with touch screens, so it should be disabled by default.
I think this is not something WebKit should support, but the toolkit. On Maemo for example there is HildonPannableArea which works fine with WebKitGTK+. Or the scrolled widget included with Midori. And there are other scrollable widgets.
Check https://bugs.webkit.org/show_bug.cgi?id=50561 , it makes PAN_SCROLLING a runtime setting.
Pan scrolling is actually a slightly different feature. When you press the middle mouse button it allows you to scroll by moving the cursor. I agree that this should be a feature of the toolkit. Touch support is just now trickling into GTK+, so we need to figure out the best way to integrate drag-to-scroll while respecting the wishes of the toolkit.
This should probably happen in the toolkit.