CDs and iTunes Plus sound almost identical. You'd need an excellent system (DAC, amplifier, speakers, room, ears) and to be actively looking for it in order to tell the difference between the two.
While that's a very valid point, and the view I used to hold up to a few months ago, it's not quite the whole story (with all due respect).
Depends on the quality of the DAC, amping, headphones and your ears. What I was saying was don't assume you need to re-buy all your music. You may find the CD versions and those bought from the iTunes Store indistinguishable. Listening via an Apogee Duet and pair of Denon D2000 headphones, iTunes Plus rips sound very, very close to the CD.
I have two listening systems, one comprised of a pair of B&W DM303s, a Rotel RSX-1055 amplifier and a BBC/Sonic Solutions broadcast DAC and on this system I simply cannot hear the difference between my CDs ripped in 256kbps AAC and Apple Lossless. On my other listening system (RME Fireface 800 + integrated DACs -> Adam A7), the difference is most definitely there on A/B comparisons, and even 320kbps AAC is a step or two down from Apple Lossless (even more so with MP3).
However, what BOTH systems show up is fatigue - listening to Apple Lossless/original CD at length on both systems is just
so much more comfortable than listening to the lossy counterparts at length. Heck, the difference is even there with my cheapo Sennheiser cans and my iPhone, even though I can't tell the difference straight away.
Listening fatigue really is all about psychoacoustics, something a direct A/B comparison doesn't always cover. Hence, I believe Apple Lossless is always the best route to go down if you really enjoy listening to music...
I was always hoping Apple would start selling Apple Lossless downloads, but now I'm convinced that the next worthwhile step up in audio quality is 24-bit and a hell of a lot less of
the other kind of compression.
I wish this would happen too - 24bit masters always seem to have more depth and precision in imaging than 16bit versions - releasing 24bit recordings will unfortunately require a remaster of the original material though

I can't see that happening for a long time, if ever.
The difficulty with any perceptual compression scheme is that everybody's perception is different. And I don't just mean in terms of what they hear and see, but also in terms of what they find desirable in any given thing. Some people add noise to recordings, where others do everything they can to completely eliminate it.
+100 to that, which is why it's incredibly important to
always audition loads of equipment before you decide what to buy.