Windows 7 Ultimate

In the retail channel, this edition will be called Ultimate; for corporate customers with a Select license agreement, it will be called Enterprise.

In either case, the feature set includes everything in Professional edition plus support for BitLocker whole-drive encryption (and the new BitLocker To Go feature, which adds high-grade encryption to removable media).
This edition also includes all supported language packs (those cost extra for other editions) and the capability to boot from a VHD.
Microsoft is de-emphasizing the Ultimate edition, which has only been able to gather a tiny share (a Microsoft told me yesterday that Ultimate's share is in the 3-5 percent range).
It will still be available, but primarily for those who want BitLocker and as a premium upgrade for super-high-end machines where the Ultimate name might add some cachet.
The real news is that each edition is a superset of the one before it. That means you can upgrade from, say, Home Premium to Professional by purchasing an upgrade key and then "unlocking" the additional features. The entire process takes 5-10 minutes, I'm told by people who've tested it, and involves none of the hassles of the current upgrade strategy, which requires a complete reinstallation.
So what happened to those other editions? They're still around, but your ability to buy them is highly constrained.

Windows 7 Ultimate is the non plus ultra edition of the Windows 7 operating system as it is the only edition of Windows 7 that is feature complete.
All other editions - with the exception of Windows 7 Enterprise which is Windows 7 Ultimate branded differently - lack features that are included in Windows 7 Ultimate.
Windows 7 Ultimate comes with the following set of features that distinguish it from other Windows 7 editions:
Bitlocker file encryption
Direct Access
Branch Cache
Plus all features of all other Windows 7 editions.