In my experience, unpaid internships aren't worth it. I'm based in London and there's a lotta dodgy startups which are trying to do too much with little experience or infrastructure to execute their plans (the whole 4/5 startups fail in their first two years bit -- "we do design, dev, marketing, strategies, management, editing and mire!" type deal), usually with one or two key guys or gals running the show who actually have some idea of what they're doing. Their objective is to build up their company or group with as little cost as possible and the dodgy places will offload work for normal employees onto inexperienced and professionally unseasoned grads who have no idea what they're getting into or what they're worth, usually for the same hours as a full-time employee and unpaid and they'll even be sketchy on reimbursement for travel or lunch.
Steer clear of smaller places that offer unpaid internships as a general rule -- there's nothing you can't learn from a book and practise out yourself that you'd be doing for one of these places. Not every startup is like this but its common enough to be a hazard to watch out for. If you really wanna do the unpaid route, get with the biggest names around. Like a prestigious university, brand value is everything and very helpful when you're first getting up the ladder. Seriously, if you're at all ambitious about your career, this should be the only reason you would consider doing unpaid work, like anything else in life.