News so far tells us that there will NOT be an editor released with the PC version of the game. There is still however a chance that this will be released seperately at a later stage. The developers explained in several interviews that it is not only a lot of work to make an editor including help files to make it usable for the general public, but they would also have to do intensive testing to make sure mods would not interfere with the game in the form it is released.

We modders know of course, that even if there will not be an official editor released, we will still be able to make mods. Many mods for Oblivion did not use the construction kit at all, but merely consisted of new texture sets and such. Keep a close eye on this site for news and tools for modding Fallout 3 !!