Hey there, I think I might be able to help you fix your problem!! I just got a 20" LG LCD HDV and I have a PS3 hooked up through HDMI. I had the exact same problem as you, the sound would gradually lag about 2 seconds behind. I figured out what was wrong, it wasn't my TV's fault. The PS3 automatically set itself to run 5.1 surround sound and my TV only has two speakers. The TV couldn't process the sound properly in 5.1, so this is how I fixed it....

On the PS3

- goto the "Settings" tab from the main menu

- then scroll down to "Sound Settings" and click on it

-the select "Audio Output Settings" click on it

- select your type of connector for you audio "HDMI, Digital Out, Audi input...." press X

- now on this screen go down to "Manual" press X

- you'll see a big list of different audio settings, the only one you change is the very top one "Dolby Digital 5.1Ch."

-uncheck the box beside "Dolby Digital 5.1 Ch."

-now simply press X and everything should work perfect. No more lag. I really hope this works/helps you.

Enjoy your PS3 it's the best machine yet :)