Inter-sample clipping is a HUGE problem.
Apple gives away software that lets you demonstrate this for yourself. It's called AU Lab and includes an AU AAC RoundTrip plugin you can use with any DAW that can use AU plugins and it is all free.
This is a fantastic tool and I urge anyone with an interest in this topic to download it and investigate for themselves.
Here's a video of the tool in action. The recording engineer for this particular recording was pretty shocked that I, as a consumer, would send him a scathing email regarding his handling of one of my favorite artists.
It would be nice if I had a Mac...do they give away a Mac?