I might be late to the party on this one, but someone sent this link around at work today and it's definitely worth sharing. Alexey Gavrilov has rebuilt the same animation/mini-app in all of the major RIA platforms that are in use today (DHTML, Flash, WPF, Silverlight, etc.) and published them for your analysis and viewing pleasure. He's also publish the perf results that he got when running the app in the various platforms. His results were very different than what I saw on my machine, but still very interesting. The results page also has a good (albeit long) discussion of the results that's a worthwhile read.
UPDATE: I don't know how he's calculating frames-per-second, but I'm getting outrageous numbers on the Silverlight 1.1 (CLR) version of the test. Results are in the range 500-600 fps which, of course, far exceeds the capabilities of my 60hz laptop monitor. Is this a fluke on my machine or is everyone getting those kinds of results?