My guess would be a Fraunhofer reference encoder. 

strings /Applications/iTunes/Contents/Resources/English.lproj | grep Fraun
gets you "MPEG Layer-3 audio coding technology licensed from Fraunhofer IIS and THOMSON multimedia."


It's certainly possible, though, that they used some other encoder, and just paied Fraunhofer and Thomson to avoid any legal entaglement with the patent holders.
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-Aaron