Arbitron's released some interesting results from its tests of Portable People Meters, which they hope will provide better cross-media and multiple-media usage measures. A limited test in the Denver area suggested that peak usage for radio and TV occur at different times - during the day (6 am to 4 pm), radio dominated, accounting for 70-80% of the combined audience, while at night (7 pm to midnight) television dominated (80% of total audience). In addition, the study showed different audience profiles.
This suggests that use of radio and TV differs enough that they don't compete that much for audiences, but rather serve largely different audiences. From an advertising perspective, that suggests that TV and radio shouldn't be seen as direct competitors, but as complements in reaching general audiences at different times.
Source - Arbitron Study: TV and Radio Augment Each Other, TVBlog