This excerpt taken from the DT 20-F filed Mar 1, 2007.
Revenue recognition for long-term service contracts
Business Customers conducts a portion of its business under long-term contracts with customers. We account for certain long-term service contracts based on proportional performance, recognizing revenues as performance of a contract progresses. Factors affecting estimates of contract progress may include total contract costs, remaining costs to completion, total contract revenues, contract risks and other judgments. Estimates relating to long-term contracts are subject to regular reviews and are adjusted as necessary.