After wanting to access a common shared Radmin phonebook among multiple workstations from one central location on the network for quite some time, I figured out this simple solution.
NOTE: This solution applies to Radmin Viewer 3.0 Beta on Windows XP.
The Radmin Viewer stores it's current phonebook data in an RPB file that, by default, is located in C:\Documents and Settings\username\Application Data\Radmin. You can move this file to a share on your network, and then modify a registry key to tell Radmin Viewer to look for it's current phonebook in this new location.
Change the path in the follow registry key to point to a network share containing your phonebook file:
After making these two simple changes, your Radmin Viewer will load it's phonebook from the network share each time it is opened. If someone else on the network makes a change to the phonebook, the phonebook is changed instantly as the user makes it, but all other users will have to close and re-open their Radmin Viewers for the change to take effect on their workstations.
This is obviously not an offically supported feature of Radmin Viewer, but it works perfectly for me, and has made keeping everyone's phonebook file on my network 100 times easier.