unfortunately time machine still doesnt work well with RAID, so my little setup here is my DNS 323 which i keep all of my mp3s, video, work stuff on and a cheap 1 TB plugged into the imac which runs time machine. hard disc on the RAID dies?? no issues, really. buy a new one, bingo we're back in action. external time machine one dies?? annoying, the worst would be a complete reinstall from the get go and a small amount of data loss. i know this may be a little dearer in the long run, but i dont really have to worry about losing my important stuff.
important question, eventually down the road when i get a MBP, is there a way of setting time machine to backup to the external plugged into the back of the imac, or do you need to plug it directly into the MBP.