Running ads in multiple channels that overlap seems like a good idea if executed well and it's affordable.  Perhaps as Google moves into this area - the results - metrics will housed within AdWords and Google Analytics, and we'll be able to measure, to some extent, the lift that an ad in one medium brings to a similar ad in another channel.

But, till then (maybe next year?) you'll need to take my word for it - if you can run AdWords, Banner and Radio ads in the same market - get the same people to look at all three - your going to get better results and better branding.

There's a good post about this in Marketing Pilgrim - Radio Ads Enhance Internet Ads.

 "..Recall of advertising is dramatically enhanced (27 percent versus 6 percent) when a mix of Radio and Internet ads is used compared to website ads alone. "

Each medium/channel is different though - you have to alter the ad to work best in the channel your running in:

"….Use more emotion-based appeals on the radio. According to the study, radio ads “may be more effective at making emotional connections with consumers, thanks to the much more emotional link that listeners have with the medium itself.”

• Radio and Internet can each be used to reach “light users” of other media, but their reach is different. Together, they reach 83% of adults aged 18-54.
• Finally, since the radio often reminds people to look up information or sites on the Internet, it’s probably wise to advertise when people are both listening to the radio and using the Internet. Peak hours for simultaneous usage for both are 6 AM to noon (32.6% of Internet users also listening to the radio) and noon to 5PM (24.8% of Internet users also listening to the radio). "

I got all of this out from Marketing Pilgrim and the sources Jordan McCollum  references.