I can't count the number of times I've seen customer requests for a "Mark All as Read" and the dangerous "Delete All" options in the Windows Mobile Inbox's menu. I personally find the "Mark All as Read" invaluable, especially when dealing w/ tons of corporate mail or mailing lists. Hopefully one day soon it will come standard out of the box.
The good news is that these two inbox menu options already exist today! The bad news is that they come only as a sample app in the PPC/SP SDK. That may explain why the average user has never seen it, since downloading the SDK, compiling it, and creating a CAB setup project is quite a pain for a developer, let alone an end-user.
I finally grew tired of flipping through all of my mail to manually mark them ALL as read, and so I went ahead and compiled the code w/ out any modifications, and threw together the CAB project. You can get it below. It should install on Smartphone 2003+ and PPC WM5.0 devices.
For those who are interested in how it works, take a look at the sample code in either the PPC or Smartphone SDK. This is the InboxMenuExtensibility sample in the Samples\CPP\Win32 path.