and the root partition certainly qualifies as "in use".
Yes, it certainly does. :mrgreen:
Resizingany partition presents the possibility of data loss, but you're definitely asking to get hosed if you try to resize an active OS/system partition. Also remember that with your particular partition scheme, all branches of your filesystem (/usr, /boot, /home, etc.) live in the root partition. If something goes wrong with the resize, you could lose everything on the drive.
One possibility is to boot from a rescue CD or a "live" CD like Knoppix and run a partitioning tool from the CD, but even that isn't foolproof. The fact that you're using reiserfs makes things even a bit more iffy, because many partition manipulation utilities (even the "pay for" Partition Magic) don't support reiserfs well, if at all.
Given that you're planning on upgrading SuSE soon anyway, I'd back up all of your critical data to CD or a spare hard drive and start from scratch. This time though, plan for expansion: slice the drive into several smaller partitions so that you have some spare space to play with in the future.