"Unlimited" space in any online service almost always means "there is a soft cap where we start threatening to shut down your service, but we won't tell you what it is ahead of time".
Actually quite an interesting point. Consider S3. They provide unlimited storage, but their software has to be able to keep up with the insane growth of data, albeit the hardware storage demand as well.
S3's cost to the consumer scales with the amount stored. That's different from Amazon, Dropbox, or Google, who offer enterprise-facing non-developer storage options at a flat rate for "unlimited".
They can do this because these enterprise-facing products offer fewer performance guarantees relative to their developer-facing products. But, ultimately, "unlimited" doesn't mean unlimited, in literally all of these products.
I would think enterprise offering would have a stricter SLA than developer-facing product. The only thing between Dropbox and Amazon is that there is no independent enterprise offering for S3, except Amazon pays a penalty for violating said SLA.
