iMac 27 Lion vs Sony TV 40BX420 - HDMI Issue

I've connected my iMac 27" running Lion 10.7.2 with my Sony TV 40BX420 using MiniDisplay HDMI Adapter + HDMI cable.

I've the following setup with my iMac & TV ... I've the same setup since last 2 years, and it always works, except this time, I've replaced

the TV, earlier, I was using a older version of Toshiba LCD TV.

Setup:

* The TV is configured as primary display with menus on TV, when connected.

* The screen was extended not mirrored

* I'd the physical hdmi+minidisplay adapter was always connected.

Usage:

* The TV was normally used in the Antenna mode (regular broadcast)

* Whenever I need to switch to iMac with TV as primary ...

- I used to change the TV's input from Antenna to HDMI 1 (connected to iMac)

- the moment the input is change the primary display of iMac + Menus & Sound were automatically

transferred from the iMac to TV

Problem:

* Now when I connect the new Sony TV to my iMac, the primay displays always goes to the TV, even when

on the TV the source is Antenna, not HDMI.

* I've no way to control the iMac, other than changing the source on TV to HDMI 1 (iMac) or physically removing

the hdmi connection, which immediately changes from 2 displays to 1 display.

I've fiddled with all settings on my Sony TV, to see why my 'changing the source input' is not changing the iMAC. I know

somehow the HDMI connection is acting as active even after I change the source. The older Toshiba TV used to work fine

with my iMac.

Please advice.

Thanks

Sajid