How do I disable secondary display connective via HDMI on Lion/Mini?
I had the same problem, this happens because the TV is still sending a signal to the HDMI port even when powered off so the Mac still thinks that it's active. To solve this you need a HDMI switcher that will fully kill the HDMI signal when it's not used. Here is the one I use:

http://www.amazon.com/Ultra-High-Performance-Switcher-Switch/dp/B0015YWKYY/ref=sr_1_1?ie=UTF8&amp;qid=1328391227&amp;sr=8-1