Thanks for the input, Anthony and Jonathan. I decided to go ahead with 
rdiff-backup, as Jonathan suggested.

My first thought, was that I don't really care about previous 
increments. But, after reading the man-page for awhile, I decided that I 
wasn't really giving anything up by using rdiff-backup over rsync, so 
why not?

I also followed the excludes listed in Anthony's script... thanks for 
that. One thing that wasn't in there however -- and the main reason for 
this post -- was /usr/portage.

I'm in the process of doing the initial backup, and I started looking 
ahead... doing du on some of the remaining directories. I was bummed to 
discover that /usr/portage uses 1.5 gigs! (In fact, the backup's been 
going for about six hours now, and I think I might be about halfway!!)

So, the question is, is there any point to including /usr/portage in the 
backup? I mean, if my server's drive crashed and I restored the backup 
onto a new drive, the first emerge sync would restore everything under 
/usr/portage, right?

Then again, I'm stuck putting it in the backup now (it's mired in 
distfiles right now)... maybe I should just leave it... might make a 
full restore easier. Still, it would be useful to know, if my backup 
space started getting full, could I skip everything under /usr/portage? 
Is it *only* the portage tree under there?

Thanks,

Ben

PS: I excluded /dev, /mnt, /proc, /sys, /tmp, /var/tmp, /var/cache/edb, 
and /var/db.