I did another search and found this article:
http://www.naavi.org/cl_editorial_04..._font_case.pdf
According to the conclusions, pt. V of this document, the Southern Software, Inc. case was legally binding only in Northern California as of 1978. I was unable to find any extension of it to the entire US, so it looks as though it's probably ok to use SSI fonts in epubs. I'm very interested in other viewpoints and urls about legality of using SSI fonts!
Bob