I'm running into the same problem as described in this thread:
http://boinc.berkeley.edu/dev/forum_thread.php?id=3540

* client is running (verified using ps)
* nothing interesting in boinc log
* boinccmd can connect to localhost
* boincmgr can't
* password is correct (even tried manual typing)
* boincmgr has read access to gui_rpc_auth.cfg (password is autoselected when connecting to localhost)
* networking is correctly set up (checked using normal and reverse lookups with dig)
* even that odd "can't determine fully-qualified domain name" disappeared after adding localhost.localdomain to /etc/hosts, yet still no improvement in connecting.

Some info:
# uname -a
Linux cami 2.6.32-gentoo-r7 #2 SMP Wed May 5 15:04:16 CEST 2010 x86_64 Intel(R) Core(TM) i7 CPU 920 @ 2.67GHz GenuineIntel GNU/Linux
# boinc_client --version
6.10.36 x86_64-pc-linux-gnu