When trying to mount a Windows share in Ubuntu 11.04 it fails with a rather useless error description, but there is some usable info in dmesg:

$ sudo mount -t cifs \/\/servername\/share_name \\
 -o username=Username,password=Password /mnt/share_dir

mount: wrong fs type, bad option, bad superblock on //bsrfile12.res.hu.corp/,
       missing codepage or helper program, or other error
       (for several filesystems (e.g. nfs, cifs) you might
       need a /sbin/mount. helper program)
       In some cases useful info is found in syslog - try
       dmesg | tail  or so

$ dmesg | tail

[90319.053420] CIFS VFS: cifs_mount failed w/return code = -22
[90408.561437] CIFS VFS: cifs_mount failed w/return code = -22

The solution is to install the 'cifs-utils' package:

$ sudo apt-get install cifs-utils