I think unblock-us.com is a little different than just a straight VPN. It's hard to tell from their site, but I think they use a combination of altered DNS data and custom proxy services to offer something that differs from a regular VPN. unblock-us only works with services (e.g. like Netflix) that they have specifically coded for.
It may be something like the following: you enter Netflix.com into your browser: their DNS servers direct you not to the normal IPs for Netflix.com, but to a custom IP address, which is unblock-us's proxy for Netflix. They apparently proxy just a part of the communication with Netflix (perhaps the authentication), providing a US-based IP to the real Netflix service. The remainder of the communication, the actual streaming, happens direct, without have to go through their proxy. The advantage is that the Netflix communication goes at the speed of your internet connection and is not slowed by unblock-us's proxy/vpn service.
More details on how unblock-us actually works would be helpful, but they are keeping the information under wraps.
This link provides a bit of information on unblockus: »www.damonkohler.com/2012 ··· ity.html