I've successfully moved my /home/andy (user) directory to a separate 180GB reiserfs mount.  It's been working great so far.  Here's my mount system:
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/sda2             45896256  37548700   8347556  82% /
varrun                 1030612       176   1030436   1% /var/run
varlock                1030612         4   1030608   1% /var/lock
udev                   1030612       184   1030428   1% /dev
devshm                 1030612         0   1030612   0% /dev/shm
/dev/sda1            188284040   2889784 185394256   2% /home/andy
/dev/sda5              5116668   4893388    223280  96% /media/sda5
/dev/sdb1            145404224 140640384   4763840  97% /media/sdb1
/dev/sdb2             47458504  36681656  10776848  78% /home/andy/storage
/dev/sdb4             43359860  37029888   6329972  86% /home/andy/storage2
/dev/hdc                716350    716350         0 100% /media/cdrom0
----------------- Migrating your /home/user dir to a bigger partition (post-installation)
Note: I have not followed the below exactly step-by-step, but I have done steps very similar to this to get mine working.  Thus, please make sure you know what you're doing before you attempt this and that you have backed up all critical data.  As long as you don't mistakenly format a Windows partition and if you have only system files (no critical documents) on your Linux partition so far, you should have nothing to worry about.  I still recommend you backup in case you or the partition editor makes an error.
Before starting, YOU MUST have an empty/unallocated/blank partition unless you have a partition full of data you don't care about!  Type gksudo gparted in the console to start the partition editor.  Format the new partition (you will lose all data previously there) as ext2, ext3, or reiserfs (reiserfs is my favorite).  Write down the new partition's device (e.g. /dev/sdb2).  Exit gparted.
With the Ubuntu user administration tools create another user like "admin2", give him an administrator template and a password you'll remember.  Then, logout of GNOME.  Get to a console by pressing Ctrl+Alt+F1.  Login as admin2.  Then type sudo mv /home/user /home/user.old, replacing "user" as necessary.
Now we are going to add the new partition to your file system table.  First recreate the directory.  Type sudo mkdir /home/user  Use a terminal text editor (as root under sudo) to add the following line to /etc/fstab (of course, replace /dev/sdb2 with the appropriate device).  Do not replace the last "user" with your user name.  "user" is an option given to the file system.  Do change the "user" in italics to your user name.  For my example I'm using /dev/sdb2 as the device.
/dev/sdb2       /home/user      auto    user         0       1
Space out the line as you wish to match the others, and save to /etc/fstab.  Exit the text editor.
Proceed to mount your partition.  Type sudo mount /home/user  Hopefully everything went fine (it will print absolutely nothing in the case of no errors).  If it errored out, make sure that partition is formatted as either ext2, ext3, or reiserfs.
It's time to copy your old data over.  Remember that first command you used to move your original user directory to user.old?  We are going to use that user.old directory now.
Type sudo cp -a /home/user.old/* /home/user/
This may take awhile as all of your data is being copied over.  Now set ownership of everything in there to you: sudo chown -R user /home/user/ (do NOT do this to anything more than your home directory).
When the commands finish, you should just reboot by pressing Ctrl+Alt+Del or if you know how, it's fine to login as the user now under GNOME.
Once you're confident everything is working correctly, delete the /home/user.old directory which is still sucking up space on your root / partition.
(If you have problems you can also login as admin2 to GNOME.)