There are three main types of testing of any application:

1. Unit testing, where you test small pieces of functionality
   
2. Functional and user-acceptance testing where developers, QA engineers and the end user can ensure that the application does what it’s expected to do
   
3. Load (or stress) testing, when you ensure that the server can process requests of the specified number of users

Rich Internet applications are no different when it comes to testing.  While the load testing software is not cheap, there are affordable solutions for Flex applications that you need to know about.

Flash Builder 4 integrates nicely with FLexUnit4 and I highly encourage you not to ignore the unit testing. Just use it.

Your functional testing needs can be completely covered by the open sourced product FlexMonkey 4, created by Flex professionals from Gorilla Logic.

They also have a FlexMonkium, which is a Selenium IDE plugin that  that adds FlexMonkey recording and playback to Selenium. With FlexMonkium, Flex recording and playback is seamlessly interleaved with native Selenium recording and playback so you can easily automate the testing of applications that mix html and Flex components. In addition to creating and running Flex tests in the Selenium IDE, you can export your scripts as JUnit tests that run with Selenium-RC.

I'd like to offer you an audio recording (MP3) of my conversation with Stu Stern from Gorilla Logic about the unit and functional testing of RIA in general and FlexMonkey4 in particular.