首先，从BlackBerry论坛中的帖子中，可以看到，PlayBook上面应该是支持用Adobe AIR的mxml开发GUI界面。
in MXML the foundation of it looks a lot like HTML but it also incorporates actionscript 3 scripts. Your main application file will be in an .mxml format. There are some playbook specific classes you will not be able to access due to incompatibilty reasons. but there are work around if you look around the forums.
The other alternative is using just actionscript 3 code and structure. Here you dont use any MXML coding and you simply use OOP style coding to get what you want. here you will run into some missing classes such as the MX packages.
There are more differences but those are the basic ones. and according to the last webinar from yesterday, both format of coding are acceptable for the offer. hope that helps and good luck!
http://supportforums.blackberry.com/t5/Tablet-OS-SDK-for-Adobe-AIR/BB-playbook-offer-MXML-vs-Air/m-p/673935
from following video, Flash Builder 4.5 + SDK does support mxml GUI dev .
http://devblog.blackberry.com/2011/01/blackberry-tablet-os-sdk-video/
其次，Adobe高人搞了个开源的东东，可以在mxml里面使用PlayBook的QNX UI代码。
Using MXML with QNX UI Components for the PlayBook
http://renaun.com/blog/2010/12/using-mxml-with-qnx-ui-components-for-the-playbook/
代码下载在github上
https://github.com/renaun/FlexMXMLAndQNXUIComponents
