I decided to have another quick look at Boxee today, whilst this is not an application that run's within the MCE UI, it is still a very promising option. The guys at Boxee seem to be taking a different approach to the integration for the online content sources. I'm a layman when it comes to application development, but it seems to me like they are not just displaying the BBC iPlayer and Hulu websites for example within their application.
The integration seems to go further and certainly looks more seamless to me, than some of our already favourite MCE add-ins. Some welcome news is there are also some inbuilt options and settings for HTTP proxies which will come in handy no doubt!
I've said it before and will say it again Microsoft we need something like Boxee native to our beloved Media Center, please please please!
BBC iPlayer in Boxee
Hulu in Boxee