The TrackBack feature for Radio UserLand has been released.
Radio supports both inbound and outbound TrackBack pings. For outbound TrackBack, all you have to do is enable the feature, and Radio will do the rest. Inbound TrackBack works similarly to comments -- a TrackBack link next to each post opens a pop-up window which displays inbound pings.
Here's a post in my Radio weblog which has a pop-up window that displays incoming TrackBack pings.
To enable TrackBack for Radio, follow the instructions on this page.