Stan is a friend and old colleague who happens to be the aac lead at Apple
This thread has lots of info on changes in Leopard ... (skuo is Stan's id, so follow those)
among other things:
A new AAC-LC codec is available in the new Mac OS X 10.5 (code named as Leopard) which will be released in Oct. 26. It has some significant sound quality improvement and bitrate mode enhancement. Since iTunes and QT use the AAC codec built natively in the OS X, these enhancements will be reflected in these Apple applications as well (doesn't matter you have upgraded these individual applications or not). You can also use the following commandline tools available in the OS X 10.5 to access the codec:
$> afconvert - perform various audio file conversions, including AAC encoding and decoding.
$> afplay - play back an audio file (compressed or uncompressed).
$> afinfo - display information about an audio file.
and
The newly released Apple AAC encoder offers four encoding modes:
Constant Bit Rate (CBR)
-- Recommended for live streaming --
This mode achieves a constant target bit rate and is completely compliant to the CBR mode specified in the MPEG-4 standard. This mode is suitable for constant-bit-rate network transmission when decoding in real-time with a fixed end-to-end audio delay.
Average Bit Rate (ABR) - Default Mode
-- Recommended for controlling file size --
A target bit rate is achieved over a long term average (typically after the first few seconds of encoding). Unlike CBR mode, this mode does not provide constant delay when using constant bit rate transmission, but this mode provides almost best global quality while still being able to strictly control the resulting file size and with less complexity than the CBR mode.
Variable Bit Rate (VBR)
-- Recommended for controlling the audio quality --
The audio signal is encoded with constant (and settable) quality and virtually no bit rate constraints. This is the best mode to achieve consistent audio quality across many files with the smallest file size to achieve that quality. It also has the lowest complexity of all the encoding modes.
Variable Bit Rate But Constrained (VBR_Constrained)
-- Recommended as a compromise between VBR and ABR --
This mode is similar to VBR but limits the average bit rate variation. The lower limit is the user-selected bit rate. Higher bit rate is adapted for difficult tracks and can generate up to 10% larger files than the ABR mode.
Note that QuickTime and the commandline tools on Mac can access all these four encoding modes. However, iTunes can only access ABR and VBR_Constrained.