The latest version from sourceforge (at the time of writing this, it is version 0.6.4) now has both app-indicator support (for Natty) and notification area support (for other ubuntu variants and versions before 11.04).
Install the application via
sudo dpkg -i radiotray_0.6.4_all.deb
then start the application:
nohup radiotray &
this will display the choice screen:
Click OK for Natty (or change the option).
Radiotray should appear in the app-indicators area of Natty.
To get Radiotray starting automatically - open Natty dash and search for
startup application. You can then add a new entry with the command line