...After having spent several useless hours trying to find an answer to this question on both Google and Metalink, and then trying to decipher stack trace output from core dump, it turns out that there was no setting in my .profile (in some cases, .bash_profile) file for LD_LIBRARY_PATH.
If Oracle was working just fine, but suddenly you come in to work and find it to have crashed...
And then- when you go to start the listener, rman, or sqlplus, and you get the mysterious error:

Segmentation Fault (core dumped)

BE SURE YOU HAVE A VALID SETTING FOR THE LD_LIBRARY_PATH

To determine this, simply type: echo $LD_LIBRARY_PATH at the UNIX command prompt. ...It should return the $ORACLE_HOME/lib directory.

BTW- We're using Oracle 10.2.0.3 on Solaris 10.

Hope this saves you more time than I wasted! ;-)