Has anyone did a blind test on iTunes AAC vs CD/Lo

Yes. You are correct 256 AAC is pretty darn good.

I'm curious to see if anyone on here, with a really high end system, can tell the difference.

Sorry for the bad grammar... Has anyone done" is what I meant

I wouldn't call my system "really high end," but I could tell the difference between AAC and Redbook. I had some extra time tonight, so I created 256 AAC versions of some lossless rips, burned them to CD, and had my wife place them in my changer so I wouldn't know which disc was in which slot. The music was Autechre and Cocteau Twins. I could tell the difference with CT almost immediately, the AAC compression sounded a little dull, Redbook had more detail and depth. Autechre was much harder to tell a difference, I had to isolate a certain spot on the first track; on the Redbook version, I could hear more of an etched sound as the synthetic effects began and ended, again there was more detail. But it was subtle. Overall the soundstage was pretty much identical between the two formats.
I burned the AAC files as audio files, I don't know if this would affect the sound, it might have, but I doubt it. I have upgraded my 128 AAC purchases from iTunes to 256, and those 256 AAC files sound very good. I am sure whoever compresses music for sale on iTunes has access to much better compression software than what I have on my laptop. I would rather have had done the test with these files from iTunes, but alas, I don't have Redbook versions of these 256 AAC tracks.

Good info. I only tested with iTunes downloads and cd. I never tried comparing a burned CD-r. Eventualli, I hope, Apple switches iTunes to apple lossless. I highly doubt it with the iCloud service coming up. I believe all iTunes material is stored on Apple's servers at full resolution. The encoding and downsampling occurs during the download process (thats what I've heard)