blackcomb and vienna are codenames as are "Neptune" and "Odyssey"(what ended up being combine into whistler)
ms has a weird prosess for developing stuff sometimes they endup totaly re-thinking a product and yet keeping the same name, win7 if based off xp/2k (older the longhorn/vista) it wouldn't use a similar/nearly identical driver model to vista.
What i understand happened was that they desired they wanted to change directions for security and how the os works internally to be more unix like, this slowed dev on blackcomb/vienna down, but insted of renaming the orignal project they just "Branched" vista off to get something done quicker(look at the ammount of stuff that was removed to get vista out when it came out, like winfs)
vista isnt a newer code base then win7, its part of 7's evolution, look at the windows version numbers for evidance of this.
windows 7
http://en.wikipedia.org/wiki/Windows_7
build 6.1.7000(higher when it comes out but 7000 is what most people have AFIK)
windows server 2008
http://en.wikipedia.org/wiki/History_of_Microsoft_Windows#Windows_Server_2008
build 6.0.6002
vista sp1 and up use the same code base/kernal version as server 2008
if 7 was a new/diffrent code base it wouldnt share the same numbering scheme as older windows versions
oh and for a little extra info
windows nt 4.0=nt 4.0(cant remmber extended build number of the last build of nt4)
windows 2000=nt 5.0.3700.6690
windows xp=nt 5.1.2600.5512
windows 2003=nt 5.2.3790.3959
trying to say any of these os's isnt based on the older one is silly, as is saying any version with a lower/older build number is "newer" then later build numbers, i remmber hearing about "blackcomb" back in the windows 2k days, god that was a long time ago, i think blackcomb and vienna stick out in peoples minds and sound cool, and thats part of why the names leaked so early.
ms has a few times started projects to slim windows up, but has alwase ended up abandoning them because it breaks compatibility with to many older apps and would take more resources to add that compatibility back to the core os, and MS will NEVER do a split with older windows app support like apple did with osx vs os9 one of ms's biggist selling points for buisness is that they can hold onto the same apps and hardware an insainly long ammount of time without having to replace anything.
as we all know, this effects the home user market as well, alot of people dont ever want to have to buy a new webcam, scaner, printer, or hell in many cases even crappy old software, i know people who still run office 95/97 dispite the fact that they are horribly out dated, when you try and get them to upgrade they dont wana spent the money, when you offer openoffice or the like, they dont wana bother because its to much work to learn where the buttons are(or some such excuse)
I think with the next big windows(after 7) ms may get smart and go minwin style and do lagacy app support via a virtual machien and allow people to just download/install prebuilt virtual machiens with older windows versions on them, so the person can just install their old apps on the windows they where built for without having to have support for said apps as a core part of the os(it would remove alot of old lagacy code and thus bugs/slowdowns if they get out the ancient app support wouldnt it?)
blah, you got me to make another huge post!!!!(see what happens when you get me talking about something im intrested in?)