Then why would you compare dynamo and memcached? Or suggest that Amazon could use an in-memory cache as the primary store for shopping cart data? It makes no sense.
If you want to be particular about ordering, then you can always use paxos to elect a master that handles everything serially and failsover.
Let's make a deal. I'll start "advancing the conversation" once you stop misleading people (ie. admit that your initial sweeping claims were incorrect).
So I'm not an expert in Google's front-ends and I'm not sure that Google stores "sessions" in the way you'd generally think of them in web apps. But usually when we have requirements like what you'd need for sessions (highly available, low-latency, highly scalable) we use Megastore:
Megastore is a layer on top of Bigtable that adds indexing, synchronous replication across data centers, and ACID semantics within small partitions called "entity groups."
If you want to be particular about ordering, then you can always use paxos to elect a master that handles everything serially and failsover.