Adobe released an early beta of Catalyst at Adobe MAX 2008. So, anyone who attended MAX has had Catalyst for about half a year. That person was stating that CS3 documents opened in the MAX preview.
To answer the question, yes. I am running the beta of Catalyst and I still have the CS3 suite and am having no problems.
Catalyst doesn't work with Flash, though, regardless of version. It works with Flash Builder (formerly known as Flex Builder). For that, you DO need the Flex 4 framework to do anything with Catalyst. (That means, you need to have the Gumbo SDK installed if you are using FB3, or have the Flash Builder 4 demo installed)
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