Google Talk is about to get one function pulled from it's fairly large list of features, at least from a third party perspective, after the search giant decided that free SMS is no longer viable. While not being removed from the Google Talk client itself, third-party applications will shortly have their access to SMS removed. Google said the decision is purely based upon cost and that they still support third-party development using Google Talk as a service. It's possible that support for SMS through third party apps will continue eventually, on a pay basis, though it seems more likely that it will simply be stripped from the API.
Google didn't specify how much it costs them to provide SMS services, only that starting tomorrow any non-Google apps sending SMS through their service, such as Infinite SMS, will be cut off. It's sad that the financial burden is too much for Google to deal with, but at the very least the company is being forthcoming on their reasoning behind it.