Instead of choosing one version over the other, cvs has added both versions to the ChangeLog file, and surrounded them with special separators to clearly mark the conflict in question. Now, it's up to me to replace this region with the text that should appear in ChangeLog; in this case, the replacement text is neither one or the other version but a combination of both:

date 25 Feb 2001

This is the thing I added myself

This is the part added by another developer

Now that I've replaced the conflicting region of the file with the appropriate text (and removed the "=======", etc markers), I can now commit my changes to cvs without any problems.