Google API: Old news.
Amazon API: The new hottie
Can you change your topic?
It might be very nice, as an ecommerce alternative, to have OACS modules that hook into the Amazon catalog, store, and distribution mechanisms.
Why Amazon and not google: Amazon has opened a lot more functions. For example, a useful google api, that does not appear to exist, would allow me to manage my adwords at google (it would be nice to have an OACS module that crawls my web log and search term queries and manages adwords at google). At Amazon, you can link to their stuff, find content at amazon, sell stuff through amazon, and on and on. (It's almost as though they wished to make money from this...)
So in addition to running google ads on your forum pages that find ads based on the topic, you could write an Amazon API that would find products at Amazon related to your topic. And then your portlet could create ads on the fly linking your site to Amazon for those products. And your ad could include bookcovers, album covers, or any amazon product art to post alongside the forum with all the image hosting paid for by Amazon.