I spent a few hours on the phone with some Overture sales and marketing people asking them about their API plans now that the Google API is out. First, Overture does have an API, and it's been in use for well over a year. It's XML based, but not a SOAP interface.
Overture charges for use of their API. I don't know exactly what they charge, and I'm sure it varies by how much you spend buying ads, but I think it's something like a tenth of a cent per operation. I was told that they charge a monthly minimum of around $2,000 to access the API. In other words, you need to be really big to justify using it.
The Overture API offers access to all the features of the web interface, according to the sales engineer I spoke with. It also offers access to the keyword suggestion and traffic prediction tools.
One of the questions I asked the Overture people was what their plans were now that their #1 competitor and top brand in PPC ads is giving access to their API for free? The answers didn't seem too promising. They are thinking about what to do, and don't want to seem to reactive to Google!? The time frame seemed at least 6 months before much happened.
I think they shouldn't worry about being perceived as reactive, and instead should get their API out there in the open, for free, ASAP...
Feel free to comment if I've got anything wrong, or if you have more information on the Overture API - I'd love to know more about how it compares to the Google AdWords API.