I've found what seems to be a problem sample for Nero AAC, that I was able to ABX without much difficulty at -q 0.5, -q 0.55, and -q 0.6 (I didn't bother going higher).
I am using the latest version, 1.1.34.2, with no options other than the -q.
I've uploaded a clip here:
http://www.yousendit.com/download/Q01GckhaMHd5UkZFQlE9PQ
To describe the artifact, the hiss-like percussion sounds are not as sharp, sort of smudged once it's gone through the encoder.
I hope this will be useful for tuning...
ABX logs:
foo_abx 1.3.3 report
foobar2000 v0.9.5.3
2008/07/19 04:21:15
File A: D:\AFX - Analogue Bubblebath 3 (1992) [FLAC] (REPHLEX CAT 008 CD)\08 - [cat 00897-Aa1].flac
File B: C:\Documents and Settings\tberg\Desktop\compress\08 - [cat 00897-Aa1].m4a
04:21:15 : Test started.
04:22:19 : 01/01 50.0%
04:22:53 : 02/02 25.0%
04:23:45 : 03/03 12.5%
04:24:13 : 04/04 6.3%
04:24:49 : 05/05 3.1%
04:25:27 : 06/06 1.6%
04:26:08 : 06/07 6.3%
04:28:00 : 07/08 3.5%
04:28:27 : 08/09 2.0%
04:29:00 : 09/10 1.1%
04:29:46 : 10/11 0.6%
04:29:50 : Test finished.
----------
Total: 10/11 (0.6%)
foo_abx 1.3.3 report
foobar2000 v0.9.5.3
2008/07/19 05:00:19
File A: D:\AFX - Analogue Bubblebath 3 (1992) [FLAC] (REPHLEX CAT 008 CD)\08 - [cat 00897-Aa1].flac
File B: C:\Documents and Settings\tberg\Desktop\compress\08 - [cat 00897-Aa1] - 0.6.m4a
05:00:19 : Test started.
05:02:25 : 01/01 50.0%
05:03:03 : 02/02 25.0%
05:03:52 : 03/03 12.5%
05:05:50 : 04/04 6.3%
05:06:26 : 05/05 3.1%
05:07:08 : 06/06 1.6%
05:08:37 : 07/07 0.8%
05:08:45 : Test finished.
----------
Total: 7/7 (0.8%)