Eclipse 3.4 code named Ganymede just came out with some new cool features. But I have tested to install both Eclipse 3.4 and the last Flex Builder 3 plug-in from the Adobe website, and they don't work together. Eclipse can't launch Flex editor when needed. So in case you search the Internet for this info like me before, you'll find it here.
I don't know if Adobe will update Flex Builder 3 plug-in to work with Eclipse 3.3 and Eclipse 3.4 or if they will bundle Eclipse 3.4 with the incoming Flex Builder 4. I personally hope that they will update Flex Builder 3 plug-in as soon as possible and launch Flex Builder 4 bundled with Ganymede.
It probably do not requires an update as soon as possible, but now with Ganymede, you can close file tabs with middle mouse button and add new projects to their corresponding working sets in the new projects dialogs (I talk about «Working sets» recently).
Update : I have reported an issue related to this problem. It's strange because previous issues have been reported with Eclipse 3.4 M7 and are closed for now. It's probably a new problem related to the final build or Flex Builder plug-in build on the Adobe website is not to date (mine is 3.0.194161)
You don't need the patch anymore, I give all necessary info in another post.
It works for me two times on different Flex Builder installation and some of you reported that it works on Linux and Windows. Enjoy the Mfranczak patch and many thanks again to him. Note that this was only tested with Flex Builder plugin, I even don't know if standalone version could automatically update to Ganymede.
We are of course all waiting that Adobe publishes an updated version of Flex Builder 3 that supports Ganymede (public access to weekly builds would be great).