Sean Christmann's spent the last two weeks developing a benchmark technology called GUIMark for investigating performance in Flash, Silverlight, Java and HTML. Here's his description of the tool:
"GUIMark is kinda like an Acid3 test on speed that’s geared towards RIA technologies. The goal was to figure out how to implement a reference design in different runtimes and then benchmark how smoothly that design could be animated. So far I have implementations in DHTML, Flex, Java, Silverlight 1 and Silverlight 2. All the results and and implementation details can be found under the GUIMark page."
Read more about GUIMark on the GUIMark homepage.
GUIMark is based on Bubblemark, another RIA benchmark. Sean talks about why he thought his variation was necessary here. During his development he also learned some cool stuff about how rendering engines work.