This is a deal of the week on the Ubisoft North America store and is priced at the usual £9.99 on the UK store.
To get this deal you just need to do the following:
1. Go to http://proxify.co.uk
2. Enter the following URL: http://emailus.ubi.com/cgi-bin11/DM/t/n8e60UyshY0FZp0uSS0Ep
3. Search for 'Assassin's Creed' on the UbiShop homepage and select 'Assassin's Creed' 'PC Download' which is about halfway down the page.
4. Should display price of $5. Just check out as normal from there.
Not sure about the DRM on games downloaded from the Ubishop or whether they can be registered on Steam as I'm at work at the moment so I won't be able to start the download myself until I get home.
The download will work fine from over here, the proxy is only needed to get to the website because it automatically defaults to the UK page when you visit the page using most UK based ISP's.
Seemed like a good deal to me though.