not the answer you want, but I don't consider online backup to be cost effective yet. Costs are going down, but I find the annual cost is still more than it'd cost to buy a good firewire drive, copy the data, and store it in a drawer at a friend's house or some other offsite location.
And think about this. How long will it take to archive 100Gb of data onto an online service? I know when I experimented with this, a 50Gb set of data took weeks to get fully backed up online. And if you ever need to recover that data, how long will it take to download it again and get it back where it's available? Again, weeks. The cost of storing the data is one cost. The cost of the time needed to make the bakcup, and even more important, the cost of the time lost waiting to rebuild the archive off of the cloud, is a killer for me.
For relatively small, temporary backups I'll use dropbox. But for a permanent solution, it's multiple copies in multiple locations on multiple hard drives.