I stilll don't like the term "spin-off" it implies that it's a lesser game. This isn't the case. The only sequel in Fallout history is Fallout 2.
I would like Obsidian to work on another Fallout.
Fallout 3 obviously isn't a 'direct' sequel but it is still technically a sequel nonetheless.
Still I think it's probably best described as either a re-write of or rather a 'spiritual successor' to the third game in the series and first Fallout spin off Fallout tactics; given it's central focus on a Brotherhood of Steel chapter that relocates to the east coast after abandoning the codex, non-cannocal status and the changes made to gameplay that vary quite markedly from the originals.
Fallout: New Vegas is technically a spin-off of Fallout 3 given it shares the same engine, object library and gameplay mechanics, etc making it part of the Fallout 3 series and era. For me the term really carries a different meaning when applied to games because in games 'spin offs' are more like sequels within a given series, while sequels themselves e.g. GTA IV are more like spiritual successors to their predecessors - closer to being new IPs then actual sequels. Really we as gamers really need to start coming up with new naming conventions for spin offs and sequels that actually make sense.