Update: If you think I should be shouting 'awesome' and similar words like most of the .NET community members, please take a walk down the path of 'licenses', something you all should be familiar with in every cell in your body, but by the look of all the different posts about this source release I can only conclude: hardly anyone has any clue whatsoever what licensing, copyright, software patents and related material really mean to a software developer. You didn't really think that by copying a class from the internet you owned the code, did you?
If you do framework development, take my advice and don't look at the .NET 3.5 BCL sourcecode. Using reflector isn't the same, you're then looking at IL being reversed engineered. Looking at sourcecode is different: it has comments, it has other layout, it has real variable names etc. etc. Also, use reflector only when you really have to. Using it is technically breaking the EULA.
Why you shouldn't look at that sourcecode
The reason is simple: software patents. People in the EU, where software patents are, fortunately, still not valid, should still realize that in other countries they do exist, and if you're writing software which could be sold in the US, don't make the mistake your code is liable to this. The jurisprudence on 'reverse engineering' is based on the fact that the people who are allowed to reverse engineer code have never layed eyes on the real code. As soon as they do, they can't reverse engineer the code anymore to their own benefit (whatever that may be, even rewriting code because it's internal in the BCL) because their case would fall outside the jurisprudence: it can be assumed they might have just copied the code instead of reverse engineered it.
Do realize that you're not allowed to rebuild that code, you're not allowed to modify it, copy it etc. You're only allowed to look at it, but also because you're not allowed to copy it, you have to forget what you saw, because if you don't forget it, and borrow the ideas in that code, you likely will step on some patent.
Take for example the new ReaderWriterLockSlim class introduced in .NET 3.5. It's in the System.Threading namespace which will be released in the pack of sourcecode-you-can-look-at. This class is a replacement for the flawed ReaderWriterLock in the current versions of .NET. This new lock is based on a patent, which (I'm told) is developed by Jeffrey Richter and sold to MS. This new class has its weaknesses as well (nothing is perfect). If you want to bend this class to meet your particular locking needs by writing a new one based on the ideas in that class' sourcecode, you're liable for a lawsuit as your code is a derivative work based on a patented class which is available in sourcecode form.
Software patents are evil and the release of this sourcecode doesn't help one bit, on the contrary. If you take your profession seriously, if you are interested in learning ideas about software engineering, instead of looking at that code, read scientific papers and learn from these. Dull crap which is out of touch with reality? Yeah right (just a few examples to get your appetite fired up ).
It's a red herring
Yes, I'm negative about this move. The main reason is that doesn't solve real problems at all. Take for example the case where you detect a bug in the BCL. You plow through the source-you-can't-touch and you'll discover the place where the bug originates and see how to fix it. You can't do a thing about it. You can't fix it yourself because you can't rebuild it. You can only report it back to MS and wait for a fix. Well, dear reader, good luck with that: even if Microsoft is willing to honor your request and patch it in a short time interval (read: month or so), you'll be handed a hotfix you can't distribute to your customers. They individually have to call PSS to get the fix, or you have to hope MS will release it publically which they've done with some fixes but hardly all of them.
Sure, be happy with your shiny sourcecode-you-can't-touch, but it's not something you would want: if Microsoft would have done this for the users of the code, namely the developers, they would have made the license less restrictive, at least that you could re-compile the code to include your own bugfixes till Microsoft would release them themselves.