Not an answer, just posting the python backtrace (which can't be done in a comment)...
$ roscore
... logging to /home/joq/.ros/log/230a829e-c07c-11e1-bdb4-00248cfb2904/roslaunch-vision-4-25458.log
Checking log directory for disk usage. This may take awhile.
Press Ctrl-C to interrupt
Done checking log file disk usage. Usage is <1GB.
Traceback (most recent call last):
File "/opt/ros/fuerte/bin/rosversion", line 76, in <module>
version = rosstack.get_stack_version(stack_name)
File "/opt/ros/fuerte/lib/python2.7/dist-packages/rospkg/rospack.py", line 415, in get_stack_version
return get_stack_version_by_dir(self.get_path(stack))
File "/opt/ros/fuerte/lib/python2.7/dist-packages/rospkg/rospack.py", line 470, in get_stack_version_by_dir
m = parse_manifest_file(stack_dir, STACK_FILE)
File "/opt/ros/fuerte/lib/python2.7/dist-packages/rospkg/manifest.py", line 346, in parse_manifest_file
return parse_manifest(manifest_name, f.read(), filename)
File "/opt/ros/fuerte/lib/python2.7/dist-packages/rospkg/manifest.py", line 407, in parse_manifest
m.author = _check('author')(p, filename)
File "/opt/ros/fuerte/lib/python2.7/dist-packages/rospkg/manifest.py", line 69, in check
raise InvalidManifest("Invalid manifest file [%s]: must have a single '%s' element"%(filename, name))
rospkg.manifest.InvalidManifest: Invalid manifest file [/opt/ros/fuerte/share/ros/stack.xml]: must have a single 'author' element
Invalid <param> tag: Cannot load command parameter [rosversion]: command [rosversion ros] returned with code [1].
Param xml is <param command="rosversion ros" name="rosversion"/>