One of the problems with trying to debug a Flex application is that the data being communicated in requests and responses between the client and the service is transmitted in Action Message Format (AMF), so it isn't directly visible in most web debugging proxies. What most people do to overcome this is use Charles, a web debugging proxy that supports parsing of AMF. I've used Charles previously with Java web applications, but I didn't particularly care for it and have been using Fiddler instead for some time.
The problem is that, until recently, there hasn't been a plugin inspector for Fiddler that supported parsing of AMF. That all changed recently when Jeongwook Oh opened up the AMFParser project on the CodePlex site. The initial version (1.0.1) was termed an "alpha" release, and I found that it provided about 80% functionality. However, after spending a bit of time with the underlying C# code, I was able to expand on that significantly. I've posted some of the code changes required in the discussion forum on the project, and will be uploading a proposed patch in the near future.