Apple has launched their “Mastered for iTunes” section. It’s a section of the store that has albums remastered specifically for iTunes from the highest-fidelity source. They are compressed to 256kbit AAC files, which is the maximum an iTunes file can be compressed at. This will lead to a slightly larger file, but better quality audio.
Apple launched the Mastered for iTunes page on their website as well, and it goes over some of the basics of preparing audio for iTunes. It explains some of the basics of their file formats and defines some of the terms and ideas behind audio engineering.
On the page, they also offer mastering tools that will let users easily encode audio to optimal settings. They simply drag and drop uncompressed .wav or .aif files and let the mastering tools do the rest. This will quickly compress the file to optimal iTunes fidelity.
Check out the page here.