Arkose: This is possible because New Vegas builds on the engine and assets of Fallout 3. While they have made many substantial changes from Fallout 3 the development time will have been significantly reduced by having the foundations already laid.
That and Alpha Protocol has been pushed back a year. It was essentially finished last year, but they obviously thought they'd make more money releasing in this overcrowded year.
New Vegas sounds very different to Fallout 3 in many ways. No doubt many assets have been carried over, but by the sounds of it, it will be a pretty unique environment.