Thanks for your comment and nice words. We're constantly working on new currencies and USD definitely high on our priority list. If you drop us an email to support [at] transferwise [dot] com we'll let you know as soon as it goes live. ;)
Can I suggest that you think a little bit about adding Saudi Riyal too? There's a huge host of expat workers there who get paid in Riyal but generally only ever spend dollars, pounds or euros...this kind of service would be perfect for them to transfer their money home.
Are you guys cheaper than Oanda? Do you plan to offer GBP->USD? (As a US citizen working in London, this is all I want.)