This is old news. The Gamefaqs forum for it has been up for a while, and the fallout wiki has a thing on it.
Fallout is going to be subscribing to the "Grand Theft Auto" system of numbering (as Yahtzee puts it). New Vegas being a new Fallout game in the "Vice City" or "San Andreas" sense, not a true sequel.
The new game is also being done by a goodly number of people from the original Fallout team, meaning it has a chance to be a lot closer to the spirit of the original Fallout games than Fallout 3 was.
Opinions are mixed, Bethesda fanboys and/or those who have only played Fallout 3 are nervous. Older gamers who played the original Fallout games (and even Wasteland/Fountain Of Dreams) who thought Fallout 3 was a *VERY* good game but not truely a good Fallout game, are looking at New Vegas with an unhealthy amount of anticipation, thinking that it will effectively be the "rea" Fallout 3 (continued by the original creators). Many are hoping the New Vegas team will be willing to push the rating to the limit and give the ESRB heart attacks while causing most of Austalia to keel over dead from shock as feared by the goverment who censored the drug use animations in Fallout 3 as "too intense" (omg! needles scare us! America is scary! *cries* aussie censor need to go make poo poo now).