Similar to the previous Portal's achievement, do you get an achievement in Portal 2 for bringing a radio along to certain points?
There is one achievement, Final Transmission, for one radio.
Specifically, test with all the garbage (Chapter 2, Test 6) has a radio in it, and one of Rat Man's dens. Take it there for an achievement.
Like the radios in Portal 1, the secret message decodes into 3 SSTV images which show details about the original Portal ARG, revealing the budget was about $100 and that the 'test subjects' took about half an hour longer than they expected.