I googled all over for this error with no results:

autom4te: unknown language: Autoconf-without-aclocal-m4
aclocal-1.10: autom4te failed with exit status: 1

It turns out that I had built an obsolete version of autoconf (2.53) and it was in my $PATH ahead of everything sane. Building and installing 2.61 (the latest version as of 2007/08/15) fixes this, and jhbuild is happily sucking my CPU and network time away once more.

You can find the latest version of autoconf (and a whole bunch of other nice toys) on the GNU FTP server.