First of all, modpost apparently comes with linux-headers and is built when the kernel source is run with a "make" command.
Secondly, building the kernel apparently requires ncurses, which was causing most of my module build scripts to fail.
Several posts are recommending moving from modpost to module-assistant for Debian (and here) ... but alas, most modules I am working with are already configured to work with modpost. :( But something to keep in mind if I am writing my own kernel modules.
UPDATE, June 25, 2008 3:45 pm: I installed ncurses using this command:
apt-get install libncurses-dev
and I could indeed build the kernel without any problems.