This is possible but perhaps tricky. You'll be using mdadm from the command line and the general steps would go like this:
1. define your current partitions as raid devices
2. define your RAID array as raid1 and tell it that your second drive has "failed" (not exactly true, but it won't remember in the morning)
3. start the rebuilding process, where mdadm will see your spare drive as a new replacement for its raid1 array and will begin copying your contents to the new drive.
In the end, you'll have all of your partitions replicated on each drive
except for the boot partition. This means that if the physical drive that has your boot partition dies, you won't be able to boot your system, but you still won't have lost any data. Alternatively, you can copy your boot partition onto a second drive, but it doesn't provide a seamless boot failover: you would need to manually edit GRUB to direct its boot to the backup drive.
It's been a while since I've done it, otherwise I'd give you more detail. This is a decent resource:
http://gentoo-wiki.com/HOWTO_Gentoo_Install_on_Software_RAIDPractice it once or twice in a virtual machine and you should have no problems. If you do decide to reinstall, you can get the k/ubuntu alternate CDs, which set it all up for you with a little fiddling in the partitioning section.
GLHF
PS. I may be doing this myself in a couple of weeks so please post back any good finds.
