I use a NAS running a form of Solaris, giving me the ZFS file system. My catalog, previews and all photos are stored on a RAID6 of 8 hard drives (RAID5 is dangerous with large drives, RAID10 isnt really redundant enough). ZFS is discussed in another thread, but the rundown is that it has built in checksumming (prevents file bit rot) and built in filesystem snapshots. Mine snapshots the photography array every 15 minutes and keeps hourly, daily, weekly, monthly and yearly snapshots of the file system state. I can go back one year (server is about a year old, eventually ill be able to go back further) and retrieve pictures deleted at that time.
That is replicated to external eSATA drives that are rotated off site. The snapshots are replicated as well.
I don't bother with online backup. I may at some point export all my photos as jpegs and back those up online but until then I don't see it as a viable enough backup strategy given my upload speed and size of my data (both photos and other important data).
I wont ever not use RAID for important data, far too much of a risk to the data. Its a vital part of data security, just like off site backups. It is IMHO completely foolish to not use both together.