A lot of people are saying the venue is unreserved seating, but some have gotten seat numbers with their orders. I guess we'll just have to wait and see when we get there.
Oh, check your order email, and then "Show complete order detail", and then click on your order number on the page. You should see your seats there. ^_^
The London "selling out" was probably the o2 Priority presale. The venue is an o2 venue; o2 are a mobile service in the UK, and they offer tickets 48 hours before public sale. So I imagine that's what sold out, because I doubt people would be getting their tickets today if it was.