"First, Longhorn, the next major version of Windows, is currently in pre-beta (which used to be called alpha, but Microsoft actually refers to as "pre-alpha," which doesn't make sense) [...]"
Note: this article is an expanded version of my comment on WinInfo.
Pre-alpha versus alpha versus beta versus pre-beta: terms often misused. Alpha testing is when you are feature-complete (you've implemented all the features you intend to ship) and are testing internally. Beta releases are once you've performed a lot of internal testing and resolved as many issues as you think customers will encounter. Asking customers to perform their daily work on the software can reveal problems that weren't found in the fairly sterile environment of the alpha test lab. It also allows you to get feedback on how easy or difficult to use the software actually is.
I've not really heard of pre-beta, but I suppose it could refer to an intermediate stage between alpha and beta test (for example, polishing installers - your in-house testers can probably cope with some more crufty installers than the users could).
MS seem to be changing a lot more between beta releases these days: some of the code is still alpha quality, or features are appearing in later betas that weren't present in earlier ones.
Longhorn is still a long way from design complete, let alone feature complete. It can only be said to be 'pre-alpha'. Build 4051 was just a dump to allow developers to get an early look at current thinking - call it a warning shot, if you like.
See also Joel: Picking a Ship Date.