Making the change to the app is very simple, according to the developer documentation:
To allow the system to install your application on the external storage, modify your manifest file to include the android:installLocation attribute in the <manifest> element, with a value of either "preferExternal" or "auto".
(See Install Location for Android Apps.)
This setting is ignored by the earliest versions of Android, which makes it backwards compatible.
I'll be publishing the SD card installable version of Net Ping soon.
To check out the Net Ping app, you can find it on the Android Market.