It is awesome news that Adobe is Open Sourcing Flex. To me, this is a significant announcement, and I am encouraged that Adobe is moving in the right direction. I hope that they choose to eventually open source all of their core technology and standardize all significant data formats. With the standardization of PDF and the open sourcing of the Actionscript 3 virtual machine (Tamarin) not that long ago, they are much of the way there.
As others have noted, this is not a half-assed open sourcing effort as other companies have tried. That they are using a well-understood open source license (Mozilla) is the most important thing to note. When we (eBay) open sourced our SDKs last year we made sure to only well-understood licenses (CDDL and MIT) so that other companies or developers didn't have to figure out the ins and outs of a brand new license. This was a bit of a pain to educate our legal department on why this was important, although they eventually got it, and now are totally on board.
So what is left to open source? The big thing is clearly the Flash Player. I would guess that their intention is to eventually open source Flash Player. But I'd also guess that they main hang-up may be a litany of patent and intellectual property issues (especially with licensed code). Flex itself was (as I understand it) developed completely by Adobe/Macromedia, and does not incorporate any third party code (that I'm aware of) so it was "easier" to open source. However, I'd also guess (third guess this paragraph) that the open sourcing of Flex is a bit of a trial run for the eventual open sourcing of Flash Player. What issues will come up? How will the community respond? How will this move financially impact us? Will it be a net positive, or a net negative? These are all questions that I can imagine are on the minds of the folks at Adobe.
So, if you want Adobe to open source Flash, contribute to and support the open source Flex project!
Disclaimer: I am an eBay employee and these opinions are mine alone. I should note very clearly that although I am under multiple layers of NDA with Adobe, I have never discussed open sourcing their technology with them, not do I have any inside information on this topic -- I'll tell them the same thing in private that I am on this blog!