Yes and No (to my understanding)
Yes - you can open the MXML up and run it
No - you will not be able to do anything in Design mode.
You will need to download the most current release of the Gumbo SDK off of opensource.adobe.com and follow the directions you find on that site to get your Flex 3.3 IDE set up to run it.
If thats the case then it will be perfect. Being able to work on the Flex Builder 3 (updated with Flex 4 SDK) and Catalyst will make life easier for a lot of people I think.
Will check it out as soon as possible and post here again.
sorry for getting back to this,
is it possible to import fxp files to flex builder 3? I cant find a way to do it. :S
You can approximate it by renaming the .fxp to .zip and then using Import > Flex Project Archive to import the zip file. That should pretty much do the same thing.
Bear in mind that the Flash Builder 4 beta is a free download if you want to try it out, however.