Hi Mediapirate, I had the exact same problem as you had. PRAM Reset, reinstalling Leopard did not work. I installed Switch ResX while connected to external monitor and made a custom resolution with the following settings under horizontal:
Front Porch - 126
Sync Width - 206
Back Porch - 332
Under Vertical I only changed:
Scan Rate to 56
When you launch SwitchResX choose the custom tab, then choose the correct LCD under Change Settings of: For me it was LCD Display (2). The choices here will be this and the external display (DELL 24... for me). Now I need a good calibration, it's still too blue. I tried someone else's zipped profile made from a Samsung display on this thread but it was way too yellow/orange. I'll keep looking, maybe even get a Huey Pro or Sypder 3.
Then hit the plus sign to add a new custom resolution. Use the settings I mentioned above.
I attached some screenshots of SwitchResX.
I saved this custom resolution, restarted the computer and it started working. For some reason, this just activates the monitor to work and it seems that switch Resx is not required after it started working.
I am going to try to uninstall switchresx and let you know if it is still working.
The LNT154U2-L04 monitor is much better than the LNT154U1-L03. I used to have a buzzing sound from the inverter which is now gone. I am really happy with this new screen.