08.19
the Flex 4, aka Gumbo, Open Iteration Meeting (recording here) dealt primarily with the work Adobe plans to do to bring parity (or near parity) between new Flex 4 components and the old Flex 3 components and the transition from Flex 3 to Flex 4.
Just to review, the new component and skinning architecture has been dubbed Spark. Halo, now referred to as MX, is the current Flex architecture and will eventually fade into the sunset (or Flex 5). That doesn’t mean Halo will go away, just that the goal is to transition new users and applications to Spark. They also emphasized that with Flex 4, you may not be able to create a pure Spark application due to lack of Spark components. With Flex 5 being the targeted release for the ability to replace all the MX system with the Spark components. Thus Flex 4 will be a boundary release, allowing a user to mix in Spark components alongside the existing MX components.
The primary driver of Spark will be Flash Builder 4 & Catalyst. In the meeting we were told that Flash Builder 4 will offer a new choice when creating a Flex project. Whether to use Spark/Halo or Just Halo. This will alter the auto-complete and code hinting suggestions. The user can still use one or both, but this will allow Adobe to shape the experience toward Spark. Thus allowing a more natural feeling transition without an forced choice that will lock a user down. Catalyst will require Spark for the output it creates.
There will also be a separation of the theme and the components themselves. There will be both Spark and MX components, and they will be named separately from the TBD (“Spark”) theme and the Halo theme. So the look & feel will be separated from the functionality. No name exists for the “Spark” theme, but after the suggestion of “Fire” or “Smoke” by Simeon Bateman, Matt Chotin replies “but we were thinking along those lines 
”
Along with this news, the presentation went through a good bit of Spark components. Adobe is working on better Spark/MX container compatibility. That may mean adjusting some MX components as well to unify them using Interfaces rather than hard classes. Each Spark component had a nice “what we’re fixing” list as well. The end goal is not to clone a MX component’s functionality but to work out the most features while keeping things simple. One example was that instead of having border styles of many components, a border container was created that would allow a border to be created around a single or many components.
Currently Flex 4 is targeted Early 2010 with a Late 2009 second beta.
Other handy links:
Differences between Flex 3 and Flex 4
What’s new in Flex 4 beta
i use first beta , waiting second beta.