I have been wondering whether "iTunes Plus" songs are of higher quality than songs encoded from CDs in AAC at 256kbps. While I have serious doubts that they would be, has anyone yet conducted any sort of listening tests to see if they are or are not?

I can't think of any logical reason why they would be...

It would be very hard to tell even if they were - 256kbps AAC is usually overkill whether from CD or iTunes. As most people have trouble ABX'ing AAC samples at 128kbps from the original, its doubtful that anybody would reliably hear differences between two 256kbps encodings, even from different encoders.

If it makes any difference to you, I've picked up a few tracks from iTunes Plus and can't hear any obvious signs of compression on any track so far (pre-echo, ringing, flanging, etc.). And believe me, I am very sensitive to pre-echo/transient smearing (thanks to one too many listening tests with castanets and hi-hats). In contrast, a lot of stuff I downloaded in 2003/4 has a blatant "watery" sound or HF ringing (as a search at HA will reveal).

I'm assuming that the question is whether the source used is better than CD quailty (higher bits per sample and freqency).

As Cygnus X1 says, it's unlikely that anyone could tell even if it was.

AAC isn't immune to everything. I abxed the A03_emese sample from guruboolez up to 350k (quality 0.85). Even at 256k there was no need to abx. 320 l wasn't hard but sounded much better. 350k wasn't 100% but close enough.

I've seen rumours about using the studio material instead of a CD as a source. This could mean 24/96 originals. I'm not sure whether it's true, how many tracks it would be (some? all?) but I'm quite sure it will be impossible to hear.

Can you give me a link to that sample please?

This only makes sense the the bitdepth, since the samplerate are 44khz (and therefore resampled)