Yes, there is now a mod, alpha but perfectly playable, that merges FO3 and FO:NV!

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You can literally get on a train, (I believe), near Yakov's Place in D.C. and travel to near Freeside in the Mojave.

They say say mods can be made compatible, but I run 104 on Fallout 3 and that might be a problem. But I'll definitely keep an eye on it.

http://www.taleoftwowastelands.com/