Tonight I added an additional external disk to my Time Machine managed backups. This leaves me with the following configuration:
After adding the second external drive to the Time Machine Backup I kicked off a backup manually. I proceeded to ignore it and do some coding. After a few hours it hadn't completed backing 40G of data. I did a rough calculation and it was averaging aproximately 300-400 kBs. Needless to say that's is quite disappointing. At first I suspected the Drobo. (I'll admit I'm not a USB fan.) After some searching I realized it probably wasn't the Drobo.
I found the following thread on Apples support forum. The jist of which is to stop Time Machine and delete the "in progress" folder using the Finder. (Being a *nix guy I tried it from the prompt but got errors, su'ed to root tried it and got errors, punted and tried it with the finder and it worked. Took a while for me, so be patient.) After removing the "in progress" backup restart Time Machine and start a backup.
After following this procedure my backup is clipping along in the 12-14MB/sec range. I think the arrangement of my machine might be an advantage. All of my data drives are on the FireWire bus, and the Drobo is on the USB. So, there is little contention when transferring data to the backup.