I wouldn't use tar to backup data that can be changed during backup, as it might lead to an unreliable backup. If you want to back up your system on-line, you should use one of the comercial available programmes, or LVM (Logical Volume Manager) for your partitions. With LVM you can make "snapshots" of a hardisk before backing up data. That way data won't change during backup.
You can find more info, if interested, here:
http://www.linux.org/docs/ldp/howto/LVM-HOWTO.html
I recall reading an article on Internet about making backups with it, but I couldn't find it, sorry :(.
Salva