I just upgraded ArcGIS Server to version 10 from 9.3.1. When I open the Flex page through the browser, instead of the dynamic legend loading, the wait for legend creation just keeps spinning.
The whole ASP.net webservice was built using 9.3 server so a new web service would have to created for 10. I'll have to see what I can do when I get some time.
I forgot I had already done this work. In the zip is also an update to the liveMapsWidget.mxml code that you may want, it is for when you turn off sub layers of a map service in the TOC those layers are turned off in the legend also.
I have been trying to get your new Rest Legend to work and I keep running into the same problem. Every time the Legend tries to load I get the same error. Please see the attached jpeg.
Open a web browser and test if the web service is doing what it is suppose to. So on the ArcGIS Server machine that you ran the setup on type "http://localhost/restLegend/restLegend.asmx" and then click on generateLegends and type in some default values like: