Although Bethesda announced the patch for yesterday, it seems that they pushed it back a couple of days, but no reason was given for the delay. The reason for this patch is probably the issues that users have been reporting on the forums, which were initially just crashing on exiting the game.
If you ask me, I don't think that's a serious problem in the first place, since I hate long goodbyes, anyway. However, users then started reporting random crashes within the game and some can’t even start it. We’d say that this is the actual reason for the delay, as they might’ve not experienced this, but we can’t say for sure.
At press time, Bethesda’s page lists Fallout 3 patch as unavailable, but when it goes online you’ll be able to download it here (scroll down to find it)