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.)