They're scanners that email the scans to an email address. Or have a small http file server you access to get at the scans on the machine. Otherwise I can't really think of any other simple way to do it.
Do you have an example of a headless scanner(not connected to a pc or connected to a monitor-less pc) that will email the scans somewhere or save it over network share?
The Canon PIXMA MX870 lets you scan and print over wifi. On a Mac you can even use Image Capture. It shows up under "Shared". I'm sure other Canon scanners have this feature as well.