If you have your remoting service running under SSL you might find when trying to use it from FlexBuilder you get an error along the lines of:

Channel.Security.Error error Error #2048:
Security sandbox violation: file://localhost/Users/Username/Flex/Projects/MyProject/bin-debug/MyProject.swf cannot load data from https://www.mywebsite.com/remoting/weborb.aspx. url: ‘https://www.mywebsite.com/remoting/weborb.aspx'”

Firstly make sure you have a cross domain file but I’m assuming you’ve already done that much, all you need to do is add the following to allow your locally running (non-secure) site to access the secure remote site by adding this:

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<allow-access-from domain="localhost" to-ports="*" secure="false" />