Basically, the Google Ad Creation Marketplace is a place for those interested in Audio Ads from Google to have the ads created for them.
The Ad Creation Marketplace is a searchable directory of professional audio ad specialists that AdWords advertisers may use to locate someone to assist them in the creation of radio advertisements.
Ad creation specialists can help with every step of the process, from concept and scriptwriting to final production and file delivery. For advertisers new to the radio space, or who are starting a new campaign, the directory provides an invaluable starting point for finding the talent they need.
There is a huge FAQ over at http://www.google.com/adwords/acm/advertiser.html.
The early tests are beginning now.
If you want to beta test these ads, visit http://www.google.com/adwords/audioads/ and sign up.
Forum discussion at WebmasterWorld.