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Whether it's viable depends on how much you trust the third-party that owns the cloud and that company's track record. If your company puts all of its eggs in that basket and they suffer a day-long outage or lose your data, your remedies could be much more limited than they are if you have local resources.
That said, I've been using Google Apps rather than MS Office at home for over a year and find it to be a good solution. I would find it harder to make that leap for my company.