A bug was recently discovered in AACGain which only effects Unix users. If a file is processed that is located on a different filesystem/partition from the current directory, AAAGain will fail. That's because AACGain uses the C-language "rename" function to change the temp file name back to that of the original file, and rename doesn't work across filesystems. Version 1.4 fixes this problem by creating the temp files in the same directory as the input file.
Windows users will never encounter this problem and can stay on version 1.3.
AACGain source is now located in the Sourceforge mp3gain CVS repository.
Also, thanks to the efforts of DarkAvenger and hardloaf, we have an improved Unix build system which has been tested on both Linux and Mac. You can download a bz2 archive here. This is just the build system files, you still need to checkout the source from CVS as noted above.
Thanks for your encouragement!
Dave