OK, so I just updated a two-weeks-old gnuradio and uhd via GIT, and
while UHD builds properly and installs correctly, gnuradio
can't find "uhd" when it goes to do a configure, because pkg-config
can't find "uhd"--and therefore doesn't build "gr-uhd".
This is a change from circa two weeks ago.
So, either Fedora 12 has done something to the default paths for
pkg-config, or the Gnu Radio build environment has changed slightly
in the last two weeks. The fix was for me to manually put
/usr/local/lib/pkgconfig into PKG_CONFIG_PATH before running the Gnu Radio
configure/make/sudo make install cycle, but I haven't had to do that
previously.