With a current git snapshot (of today) compiz hangs when started with:
$ compiz --replace glib gconf
on ubuntu. 

I gathered the following information:
$ strace -p `pidof compiz.real`
Process 21113 attached - interrupt to quit
read(5,  <unfinished ...>
Process 21113 detached

$ cat /proc/`pidof compiz.real`/cmdline
/usr/bin/compiz.real --ignore-desktop-hints --replace glib gconf

$ gdb -p `pidof compiz.real`
(gdb) where
#0  0x00002b0351dd34f0 in read () from /lib/libc.so.6
#1  0x00002b03538331a4 in g_main_context_check () from /usr/lib/libglib-2.0.so.0
#2  0x00002b03535e0cf0 in ?? () from /usr/lib/compiz/libglib.so
#3  0x00002b03535e0f84 in ?? () from /usr/lib/compiz/libglib.so
#4  0x000000000040d788 in ?? ()
#5  0x000000000040f50c in eventLoop ()
#6  0x000000000040c246 in main ()

It prints on stderr (I added the debug output for the plugin loading
to get a better idea what is going on):
GLX_EXT_texture_from_pixmap is not available with direct rendering.
GLX_EXT_texture_from_pixmap is available with indirect rendering.
GConf Error: Type mismatch: Expected `float' got `int' for key /apps/compiz/plugins/decoration/allscreens/options/shadow_radius
/usr/bin/compiz.real (core) - Info: loading: /usr/lib/compiz/libglib.so
/usr/bin/compiz.real (core) - Info: loading: /usr/lib/compiz/libgconf.so
/usr/bin/compiz.real (core) - Info: loading: /usr/lib/compiz/libswitcher.so
/usr/bin/compiz.real (core) - Info: loading: /usr/lib/compiz/libannotate.so
/usr/bin/compiz.real (core) - Info: loading: /usr/lib/compiz/libscale.so
/usr/bin/compiz.real (core) - Info: loading: /usr/lib/compiz/libplace.so
/usr/bin/compiz.real (core) - Info: loading: /usr/lib/compiz/libinotify.so
/usr/bin/compiz.real (core) - Info: loading: /usr/lib/compiz/libkeybinding.so
/usr/bin/compiz.real (core) - Warn: Unable to parse XML metadata from file "keybinding.xml"
/usr/bin/compiz.real (core) - Info: loading: /usr/lib/compiz/libzoom.so
/usr/bin/compiz.real (core) - Info: loading: /usr/lib/compiz/libwater.so

(process:21113): GLib-WARNING **: g_main_context_prepare(): main loop already active in another thread

If there is anything I can do to help debug/fix this, please let me know.

Thanks,
 Michael

P.S. The original ubuntu bugreport is:
https://bugs.launchpad.net/ubuntu/+source/compiz/+bug/119199