I too think PostgreSQL should be in there... and I can't see why it isn't given that Amazon have the EBS already and are probably putting Oracle on that already. The same technique could easily take PostgreSQL.
Note: EBS = Elastic Block Storage which is Amazon's low level raw block storage solution: http://aws.amazon.com/ebs/
There's a lot of companies with licenses and who have been scaling vertically and are approaching their max. The traditional ROI race means that they did their planning to minimise their licensing costs whilst maximising their hardware usage... basically companies plan for just what they need and barely leave themselves 10% for growth.
What then happens is that when they look like they're going to exceed their 10% spare capacity, they're faced with disproportionally high costs to continue to scale vertically by adding RAM or CPU when the box may already be maxed or not be able to meet demands.
Where this helps, is that Amazon can extend your LAN over a VPN to include an Oracle instance hosted by them. They can take away your capex and turn it into an opex in which you weren't paying for unused spare capacity. They can also liberate you from that expensive capex cycle.
It's a good thing, but I doubt anyone is going to be building new software this way, especially amongst the HN crowd (unless this really is 1998/1999 again).
