I guess they don't figure on getting quite as many hits as with the new Office beta ;)
Anyway, here's an Ars Technica thread dicussing the merits of the move with a link to the MSDN download page.
Suddenly M$ are getting more generous. First Visual Studio, then Virtual PC, now this. Do they have anything left to give? Hopefully we will see more of this new side to M$. When will we see a free Windows?
I'm guessing that they figure that if they give away all this stuff it will make it easier for smaller developers to produce software for Windows - potentially for free or as open source. This would then make their OS more competitive against Linux.