Update: Though I'd vastly prefer a GUI that actually showed me the waveform and let me select with the mouse and cut, the command line mp3splt program does basically what I needed (mp3splt -s -p rm foo.mp3). And thankfully, "port install mp3splt" worked. I wish it preserved the id3v2 data, though.
Anyone know where to find a build of Audacity that works on Intel Macs? Or any other program that will let me do rudimentary cut/fade/save on an MP3 file?