thunderbolt as a video connection

Has anyone used Thunderbolt to connect a second monitor to a new iMac? I got a new 27" iMac, and am using a cable i got from the Apple store to connect a second monitor, a 24" LG.

It works, but it has some issues.

When the computer goes to sleep, anything open on the second monitor moves to the main screen.

I asked at the Apple store about this, and the tech guy said there is a file in the library section (Go + option key shows the library in the menu) for this monitor that wasn't right, and to delete it and restart the computer. the file was

com.apple.(LG).plist

I went to look for the file, but there wasn't one in the library.

Any suggestions?