This was just announced today. A new Fallout game, full-fledged, not a download expansion or anything. It's being developed by.....not by Bethesda, but by Obsidian.
VG247 » Blog Archive » Fallout: New Vegas announced in London [Update]We're just heard that Bethesda's announced a new Fallout game - titled Fallout: New Vegas - at its London showcase.
Apparently it's the same style of title as Fallout 3.
We're waiting on more detail.
Update: It's releasing in 2010. Obsidian's developing.
It's definitely not a rumor, despite what the link says.