It's probably one of your pulleys. Obviously it's not the harmonic balancer since you got that fixed and the noise remains. I'd give a look at the AC pulley. There's a bushing in there that has been known to wear out on the LS engines. Turn your AC off and listen for the squeak. If the squeak disappears, then you know where the problem is. If it doesn't, there's only about 5 more pulleys it could be. Worn out water pumps make a squeaking noise too.
If it's a pulley issue, many owners notice this noise after changing belts or increasing the tension on the belt. If a GM dealer has looked at this for you and can't diagnose it, the forums are definitely the place to be. I'm sure you're not the only one who's had this problem.
If it is in fact a valve train issue (cam, timing gear, etc), you're going to have to open up the engine. I'd suggest a DOD delete, new cam, new lifters, springs and pushrods. Might as well get it all taken care of while the engine is open.
Best of luck. Pulleys are a cheaper fix.