I'm not familiar with unblock-us. Would a VPN service work, or have they figured out how to block those too? I would imagine that to do that, they'd have to keep lists of IP addresses associated with the VPN services they want to block.
Yeah, that's probably what they're doing. It wouldn't be too hard for them to get the first 95% either - beyond ARIN/NIC registration searches etc, they could just look for IP addresses that often have a large number of subscribers behind them, especially a large number of non-us subscribers.
A normally configured proxy and VPN usually end up with a flagged IP, as I've read multiple reports of people trying with DO or Amazon EC2 nodes.