Adobe quietly open sourced the Data Visualization library in March. Last I checked, they never bothered to update the official download pages to reflect this. You can however find the source in the SDK's SVN source repository if you need it (very helpful if you intend to do any customization!). The library still uses Flex 3 components. Flex 4 packs a new UI component library, but still has the older components as well since a lot of stuff hasn't been converted yet. In short, not a big deal. Should work just fine.
Open source announcement:
Data Visualization library:
Flex SDK SVN repository:
Flex Data Visualization Developer's Guide: