Following up on our earlier report about AAC encoding not giving the expected results in iTunes, it now appears (thanks to a helpful reader email) that this neither a bug in QuickTime nor a bug in iTunes; in fact, it's not a bug. What's happened is that the meaning of the choices you make in iTunes has changed (without, as usual, any explanation or notice from Apple).
Here's how things used to work. In (let's say) iTunes 7.1.1 with QuickTime 7.1.6 on Tiger, when you tell iTunes in its Advanced > Importing preferences that you don't want to use VBR (variable bit rate) encoding, you get CBR (constant bit rate) encoding.
But using iTunes 7.5 with QuickTime 7.3 on Leopard, the very same setting in Advanced > Importing has changed its meaning. It gives you ABR (average bit rate) encoding, not CBR.
According to a thread over on hydrogenaudio.org, QuickTime 7.3, the AAC encoder has been improved. As part of that improvement, there are now four encoding modes:
ABR. This is new in Leopard, and is what you now get in iTunes when you uncheck the VBR checkbox.
VBR_Constrained, a form of variable bit rate where the variation in bit rate is limited. This is what you now get, and apparently have always gotten, in iTunes when you check the VBR checkbox.
VBR. This unconstrained, low-complexity form of VBR is actually new in Leopard.
So the confusion here is caused mostly by the failure of the iTunes interface. There is no iTunes interface for obtaining CBR AAC compression any longer, but the interface does not make this fact clear. And to add to the confusion, iTunes is not (and never really has been) very good about telling you the specifications of an AAC file after you've created it.
To obtain CBR AAC encoding, by the way, you can now use the afconvert command-line tool in the Terminal. For example, this might work (though not thoroughly tested here):
afconvert -f 'm4af' -d 'aac ' -s 0 -b 128000 mySoundFile.aiff