Adobe already released Flash Builder 4.0 beta.
I've just installed it and tried first project. I see that swfobject library is using in the index.template.html file. Design mode is slightly different from Flex builder, but Properties panel and Constrains are in same style. I found one interesting panel - Appearance, on this panel now we can change global application style and whole application theme, I found here 8 predefined adobe themes. The full list of new features you could find in Matt Chotin's post.
Than, I have tried to import simple flex project into Flash Builder and it works fine. So, seems to me, it is time to move all my projects to Flash builder. I hope there won't be any serious problems.