This isn't too hard, if you have the patience and hard drive space.
You'll need:
* a tape player with an earphone jack -- preferably one with auto-flip
* a Griffin iMic
* a patch cord
* SoundStudio (it came with an older version of OS X that I have; not sure if it does now), or that app that comes with the Griffin iMic that I'm blanking on now
You'll do:
1) plug one end of the patch cord into the iMic. Plug the other end into the earphone.
2) hit record in SoundStudio.
3) hit play and make sure it's not too loud
4) when the tape has stopped, hit stop
5) save the file as a .aiff or .wav
6) import into iTunes and convert to MP3.
Ta-da!
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