This is a port of Will's VBRFIX commandline program for Mac OS X. I've only tested it on 10.3.6 for PPC machines, but it should be fine. Note that Will's site seems to be down, so I'm not sure what the current state of the code is.
You can download the PKG here: VBRFix_Mac.pkg.zip
If you're curious what the actual problem was, it was PPC vs. x86 endian coding. Here's a sample pair of screen shots to illustrate: notice after the XING header start, the values to describe the bitrate are swapped.
Windows/x86:
Mac/PPC:
