I had the same problem with the cartoon network site.
It's when you click on the cartoon network video link.
The last time I tried it (under windows), it told me that Firefox was not supported, so I tried IE7 and then it told me "Sorry, this service is only available in the U.S".
I was so frustrated that I wrote them an e-mail.
At least they seem to have improved on the firefox compatibility problem. ^^
But I don't want to bother installing activeX support on firefox under Linux right now...
I really hope that soon TV-channels from all over the world will be easily available over the internet.
I don't care if they have unskippable ads or if the resolution is as low as on youtube.
Because I already enjoy watching TV-shows on youtube. :P