Here are some answers that I received from Aaron Stebner
1) How MCE choose the online-spotlight location (In XP, we could change the system locale, what happen in Vista), Is there a way to change it?
I believe that the same algorithm is used in Windows Vista for selecting the Online Spotlight location. Developers should not need to worry about changing locales at all for Online Spotlight scenarios though. If a developer is creating an Online Spotlight application, they can work with the Windows Media Center partner team to coordinate which locale(s) the application should be offered in.
I am not sure about this answer, I have understood from other people in Microsoft that the Media Center choose the Online Spotline server by the machine IP address, but I have no official answer yet.
2) How can I test an Online-Spotlight software? (Is there a beta site that we can upload a software and set the MCE to get the online content from there?)
Developers who are working on applications that will appear in Online Spotlight should work with the Windows Media Center partner team for beta testing applications before they are released live on Online Spotlight.
3) Can I mixed an ActiveX control in MCML application? How? Can I use flash in MCML application?
No, you cannot embed ActiveX controls in an MCML application.
4) Can Flash applet read all remote control keys?
Hosted HTML applications can listen to and handle many but not all remote control keys. You should be able to find a complete list in the Media Center SDK.
5) Does XBAP application hosted in MCE have the same IE hosting sandbox restrictions?
Yes, my understanding is that the same restrictions apply in IE as in Windows Media Center.
6) I understood the XBAP app and HTML based app will run on extender using Remote Desktop technology while MCML based application will be rendered on the Extender. Am I correct?
Yes, you are correct here. MCML applications are currently the only ones that render in full fidelity on Extenders.