I have my own mod/DLC that I've been working on for 19months. The idea of having all that work depricated by the pending release of New Vegas was crushing to me for some time, and I spoke with Elminster about this matter. The really Good news is that he plans to update Fo3Edit to work with New Vegas, so we will have the power of that tool to help make the conversion from Fo3 to NV. Without him and Fo3Edit I feel that converting complex mods would be impossible. I will be making many sacrifices to him in the coming months!
For models/textures, he suspects that in addition to them re-using many models from Fallout3, that many of the FormID's might even be the same for those assets. I then went through all of the available trailers and identified over 100 items that are direct transfers from Fallout3 - stuff like road pieces, trash cans, sidealks, etc. This gave me great hope that at least some percentage of our cells could be directly converted from Fo3 to NV format, and leave us with enough left to replace the parts that don't transfer. If the FormID's changed, then the work of conversion will be a bit more tricky as a conversion table will be required that matches Fo3 assets to NV assets using the same models/textures. From that list it should be possible to directly swap the old FormID with the NV ones via script or Fo3/NVEdit. So how quickly/simply cells can be converted much depends on wether the FormIDs maintained their original names or were changed. I actually suspect both will be true, with any directly-ported asset having the same FormIDs and anything they changed/modified from the original with a new FormID - but we'll see. Much will depend on what we find in NewVegas.ESM.
I've also wondered about the DLC assets from Fallout3, as most of them contain "DLC" in the FormIDs and their assets are placed in directories like /Data/DLCPitt or /Data/DLCAnch. I have no confirmation, but I suspect that very few DLC assets got carried-over into NewVegas, as they would all require new FormIDs and to have their assets moved around at the file level. I'm also willing to bet that Some select DLC models/textures got re-used where a specific model/asset looked good for a particular part (such as the ScaffoldKit), but for the most part I don't think the DLC assets will be in NewVegas. I could be GECKulating wrong here, but playing it safe I've taken the time to strip-out all of the DLC assets from my mod with a few exceptions, and detached the DLC masters from my mod so that only Fallout3.esm remains. I GECKulate that this will be required for everyone, as a conversion makes replacing Fallout3.ESM with NewVegas.ESM, and vegas won't have the DLC masters. This clears the way for New Vegas's own DLCs. Any DLC asset in out mods will thus not work right if I'm correct here, unless they get built as new FormIDs/objects.
As CCNA said, scripts and anything custom that we have made can be ported over without quesetion so long as they use any asset only in NewVegas.ESM. Part of what I'm doing (the longest part actually) is to get my models/textures placed into a unique directory structure that isn't part of any existing/Fallout3 directory, such as /Data/Miax/Architecture instead of /Data/Architecture. This will allow me to make my own BSA's that won't conflict with Any directory that the New Vegas models/textures will use. Most experienced modders already do this, I'm getting my education now.

The result will be a set of BSA archives that will port directly to New Vegas without issues. Sounds, Voices should also be similarly packaged for a smooth transition.
As for game settings, I suspect some will port directly without any modification (perhaps the Whether is like this), but other systems like Combat and Weapons will have dramatic changes that may result in changes to global variables, control knobs, etc that will make combat-mods difficult or impossible to port directly.
For Quests and Dialogue I feel better here, as we've heard from Chris Avellone in one of the interviews back in July that the dialogue system in the current GECK will still work, but that there is also a New dialogue editor that gives advanced features. From this I GECKulcate that quests and dialogue will be directly-portable from Fo3 to NV mods, after which we can go back and use the new dialogue editor to add/change things. I'm REALLY hoping this is the case, as quest/dialogue is complicated and time consuming to build - I've got one quest with 184 users and complex branching structure that I do not want to re-create!!
For the world, Elminster and I both suspect that all but a few test cells would have been deleted, all but a few sample NPCs and Quests would be deleted and any asset that is east-coast / Fallout3-theme specific would have been deleted before starting. This would have given Obsidian the GECK with all of the generic, non-Fallout3-specific assets left in to start working with New Vegas. I think it was very smart on both companies as Obsidian didn't have to waste time making thousands of common objects, and could instead focus on building the world and making NV-thematic-specific content. This makes the 18-month time line much more understandable, and it was a true gift to the modding community as at least some or most of our mods can be converted legally and within the rules set out by Bethesda and Obsidian.
All I can say is that October 19th can't come soon enough! Until then I'll be converting and preparing my my own mod for conversion, and praying very hard that Many/Most of the Fallout3 assets (especially the kits) get included in New Vegas. :fallout:
Miax