Gert-Jan van der Kamp has posted a very nice end-to-end example on CodeProject, showing how to create a WCF service and Excel-DNA add-in to stream real-time data into Excel.
The example uses to use the Reactive Extensions support in Excel-DNA v. 0.30 to push the data to an Excel UDF (using Excel's RTD mechanism behind the scenes), together with a Duplex WCF service providing the data.
There was also this CodePlex discussion about the Excel ThrottleInterval option, which trades off the real-time update frequency against stability of the Excel calculation.