While they talk about "unlimited" storage, it's actually set to 1,000 GB: all the quotas throughout the UI are set at 1,000 GB, and you have to contact them if and when you want to go over that amount (they do mention this stipulation on the features page[1]).
I've gotten close (~850 GB), but never over so I'm not sure if it's a "no-questions-asked, 1,000-GB-is-just-a-sanity-check" type of conversation, or if they start to get more hands-on and want a really good reason for going over.
I'm fairly certain they'll double it or more the minute you ask them, maybe a couple of times before scrutinising it. Posterous had the same thing btw, with a limit of 3 blogs, and they increased my limit in a few hours.
I've implemented the same kind of workaround, and for two reasons.
It's actually a lot easier technically. I mean DB already has a limit for free users, so all they have to do is rev up the limit and all their clients "just work". They don't have to add magic if-then logic to change how clients show and enforce the limit.
Secondly, it prevents a disaster scenario where some script - either malicious or accidental - smashes your monthly quota with 1s an 0s. Yeah you could try to rate-limit it or something, but it's just easier to prevent the whole scenario by changing a database column.
It might be similar to the discussion a few days ago regarding tarsnap. Like, unlimited, but if you're using more than 1 TB, they start asking why you need that much space.
I don't make software service purchasing decisions for any companies, but I am always irritated by claims of "unlimited" for things that are very obviously not unlimited. "Unlimited vacation days," "unlimited cell phone data," "unlimited storage space," etc. all seem to me to be precisely what you said they are: ways of obfuscating the limits. You're not going to keep paying me if I take a 2 year vacation, and you're going to cut me off pretty quickly if I start piping /dev/random into Dropbox.
