Flex Builder, due to the fact it is built on eclipse, is easy to combine with various other eclipse based IDEs to provide a central hub for all of your development needs. I personally use a vast array of views and perspectives that I either mix in with Flex Builder or have open in the Perspective bar within the IDE itself. For the most part this is a nice harmonious arrangement and they all play nicely together.
However I have come across one conflict that breaks the new CSS Design view in Flex Builder 3.0. It appears that if you have OxygenXML and Flex Builder installed within the same eclipse IDE this heady combination stops the CSS editor from displaying anything beyond plain text content. I haven’t fathomed out exactly where the conflict lies but it does appear to be related to these two applications. I intend to explore further to see if I can resolve this, but if it’s happened to you this may be the reason.