Found it: IField has become IWebPartField
Recently, I posted a question about what happened to the IField, IRow and ITable interfaces that were available in the System.Web.UI.WebControls.WebParts namespace in .NET beta1 and the CTPs. I found it: IField has become IWebPartField, IRow became IWebPartRow, etc...
Interestingly, you are now supposed to pass in a delegate when you retrieve the value from the connection provider. I suppose that this is a mechanism to alert the consumer of changes occurring after the retrieval of the value? Documentation is not available yet.