It is often said that modders beat the original devs. How much truth to this is it and does it basically mean that a good "forum modder" can join Bethesda?
"Beat" is maybe a subjective term. There are some modders that are quite competent, some become moderators here on the forums, while others become game developers and work for Bethesda (The author of OOO mod, Oscuro's Oblivion Overhaul, has done some work for Bethesda). Modding gives some experience with the gaming industry, and good mods can make big waves (like the overhaul to how Archery works in Skyrim is based on a mod for Oblivion).
As for your original question, there will be some mods that come out within a few days of release. These will mostly be small bug fixes, god mode mods/cheat mods, and maybe a simple few functionality changes (an http://www.newvegasnexus.com/downloads/file.php?id=34912 by me, I have no real experience with the GECK, but I released this 2 days after the PC version because it bugged me). Simple things like this will be out fairly soon, while bigger mods by the more dedicated professional individuals and teams will come out months or even years later (the large overhaul mods that make lots of rolling changes, additional content, ect, mostly)