I think this is the solution
My phone had the screen problem almost right away. I was about to return it but decided to try out the usb tethering first.
you have to download the USB driver and LG mobile support tool to get usb tethering to work. During the install, the phone checks for a software update and updates the lucid automatically.
I have not had an issue with the screen going dark during calls since that update.
I joined the forum just to post this solution, so I cannot post links yet, but if you search for LG VS840 USB modem driver on google, you should find the right page.
hopefully a few other users can try this and see if it works for them too
June 1 2012 update:
Unfortunately, I just received a call where I had to use the keypad to confirm an appointment, and it would not light back up after taking the phone from my ear. I really like the phone, but its going back today or tomorrow.