I wrote the headline to make it clear that this was coming from a single source. I am not an expert in hosting or infrastructure.
That being said, there is an interesting discussion going on over at Betabeat. To quote on comment = "At one side is platform as a service or managed platforms on top of EC2 -- Heroku, EngineYard (which Groupon appears to use), etc. These are easy to set up, and have lower overhead for small or medium organizations. They have the highest per-unit cost, but for rapidly growing or small organizations, can be worth it."
I think its fair to say Groupon is a large company and past the point of its most rapid growth. To me that fits with the idea that they are looking to make a transition but have not accomplished it yet, whether that change is moving to EC2 or building their own data center.