Blackcomb has been scrapped. Longhorn was originally supposed to be a point release to follow XP, however MS decided to synchronize their product releases like they did with the release of Windows 95.
Now, Longhorn will be NT 6.0. All features slated for Blackcomb will instead be included in Longhorn. The technology for the Yukon database file system will first appear in the next release of SQL Server. This will then be worked into Longhorn along with a bunch of other tech that's been in the works for years.
MS has always used codenames in their unreleased products.
And, I believe XBOX is an example of them using a codename as the product's final name.
The final Longhorn release will have little resemblance to what is shown in the screenshots that were distributed on the web. This is an Alpha build. MS always uses their previous release to test new features that will be in the new release. This allows them to do such things as develop and test out new technologies, APIs and interfaces with a stable platform while the low-level teams rewrite or improve things like the kernel. If you can find old screenshots of Alpha and early beta versions of XP, you will find that it looks like Win2k and even includes Win2k references and icons. UI and other visual changes, though worked on during the entire development process, are integrated into the OS late in the beta phase. Unlike the transition from Win2k to XP, which included many kernel and other improvements, Longhorn (last I heard) is supposed to be a complete rewrite of the OS along the lines of how NT was created from scratch during the Windows 3 timeframe.
Technology improvements for Longhorn include, but aren't limited to the Yukon DB file system, integrated .NET and conversion of many, if not all APIs to managed code, acceleration of GDI+ through DirectX, support for greater than 32-bit color, UGA, Palladium, IPv6 as the default (the stack is currently included in Win2k and XP), improvements to speech, handwriting and other UI technologies, greater UI customizability, more task-based interfaces, and more dynamic content provided to the end-user, and the usual things like greater device support, and support for emerging technologies.
You can find out some of the things going into Longhorn by looking at Powerpoint slides and other info at
http://www.microsoft.com/hwdev/futurepc/winhec2002/default.asp