The rosinstall docs [1] tell me to update a rosinstall tree by doing

rosinstall ~/workspace

where ~/workspace is the location of my tree. Right now I have two
such trees, one for c-turtle and one for unstable. In both cases I get
the following message:
---
$ rosinstall .

need to specify a URI, unless you're doing an update.
---

I'm also a bit puzzled since the help for rosinstall says:
---
$ rosinstall -h
Usage: rosinstall [options]

Options:
  -h, --help            show this help message and exit
  -u, --update          update an existing installation (URI not needed)
  -s SETUP_FILE, --setup_file=SETUP_FILE
                        create environment using an existing setup file
  -o, --overlay         use current environment to build setup file
  -n, --nobuild         skip the build step for the ROS stack
  --rosdep-yes          Pass through --rosdep-yes to rosmake
---

.. and if I instead use the -u option:
---
$ rosinstall -u .
rosinstall is updating ros
D    /home/bouffard/ros_unstable/ros/tools/roswtf
D    /home/bouffard/ros_unstable/ros/tools/rosservice
D    /home/bouffard/ros_unstable/ros/tools/rosparam
...
D    /home/bouffard/ros_unstable/ros/core/roslib/src/roslib/distro.py
D    /home/bouffard/ros_unstable/ros/core/roslib/src/roslib/masterapi.py
U    /home/bouffard/ros_unstable/ros/core/roslib/src/roslib/network.py
U    /home/bouffard/ros_unstable/ros/core/roslib/src/roslib/gentools.py
svn: Failed to add directory
'/home/bouffard/ros_unstable/ros/core/roslib/src/roslib/msg': an
unversioned directory of the same name already exists
ERROR: Failed to install tree 'ros' using command:
	svn up  /home/bouffard/ros_unstable/ros
Perhaps there is another installation in the way, or there are
permissions issues.
---

What am I doing wrong here?

The same command (with the -u) does, however, work on the c-turtle tree.

Also a related question: after doing rosinstall -u can I issue a
single command that will rebuild all the packages that had updates?

Thanks,
Pat

[1] http://www.ros.org/wiki/rosinstall