Mastered for iTunes (MFiT) is less complex than it seems. Click here for Apple's documents and droplets for diving deep. I recommend reading them all. But here's the short version:
Apple figured out a way to make AAC encoding sound better in some ways than CD. To do it, Apple has to feed the encoder mixes that are at least 44.1Khz/24-bit. 96Khz is preferred, but 44.1 is tolerated. Must be at least 24-bits. When you're close to done mastering, you run your mix through afclip in the Terminal app, which will tell you if your mix will clip on AAC decode (the decode happens when users are listening to iTunes or their iPhones).
After some experimentation, I found that I needed to leave at least -0.7dbfs headroom on my mixes, and sometimes even more (I did -1.5dbfs recently which is a lot). This is contrary to the old CD-mixing advice which was to push the mix way up to the ceiling, like -0.3dbfs. Those mixes will fail MFiT.
You can also "round-trip" your mix by using the AAC>WAV droplet and sending the file back into Pro Tools. This will let you A/B the process. There you can cancel the two mixes and then really hear what AAC is (AAC always sounds like a swishy fish tank to me when I do this, but a good swishy fish tank).