Here's a weird bug. I connected my macbook pro to my Samsung widescreen TV - it was detected but I had to select 59.9 as the refresh rate (rather than then autodetected 60) otherwise the TV just sat there with a message that I was using an unsupported display setting. That's fine but how to do this in clamshell mode as macs seem not to share monitor settings between dual monitor mode and clamshell mode (or for that matter a mac mini connected to the same TV)?
My solution was to turn on screen sharing and enable VNC access. Then I plugged the mac (in clamshell mode) into the TV and connected to it via VNC. I could then select a 59.9 refresh rate and the TV magically came to life. This only needed to be done the first time I connected the mac & TV together. Weird ....