After attending a seminar on long term data storage for broadcasters a couple of years ago, the only solution they were willing to recommend was tape, more specifically LTO. The guy gave specific examples of customers that had bought a disc, filled it up, stuck it on a shelf and a year later found the disc wouldn't spin up.

Of course, this recommendation was based on the fact the data these people have was 'worth' anything from tens of thousands of pounds to millions. If the long-term survival of my business depended upon the integrity of my backups then tapes in a fire safe seems to be the only solution. Otherwise, if you're only holding onto the data as a courtesy (non-contractual) service, then buying a load of discs may be the answer. But I'd definitely store them off site.