Don't think this has come up on the list yet, but you can have LaTeX number
your subsections using "A,B,C" rather than "1,2,3" (in the same way as
in
the problem set), using
\def\thesubsection {\alph{subsection}}
in the preamble.
More information:
http://help-csli.stanford.edu/tex/latex-sections.shtml
Ari