It just takes one person that has access to the code (and there are thousands) to give it away for 30 minutes and it's out. To put things in perspective, the Vista RTM was wednesday, but the final build was from last week (it just took so long to get it completely
2. can modify it so fast
Vista is not that different, in architecture, from XP. A decent coder, familiar with XP, can find his way around Vista pretty quickly.
3. have it on line so fast
Automatic splitters and uploaders are very fast these days
At the same time, Microsoft has to run the final test runs, synchronize all the KB and the download pages, put the code on the staging servers, verify that everything works as expected, push the release to the production server, let it propagate on the server
When we released WDS 3.0, it took three days to get it publically available. You can imagine that Vista is slighty bigger