I suspect there are certainly still a few barriers to large corporates adopting the cloud but the introduction of VPC (Virtual Private Cloud) certainly lowers one of them.
This capability allows organisations to essentially create a satellite network that is linked via industry standard IPSec to their own network. As with most features this is an added cost, nothing up front and based on a utilisation model.
The capability allows architects to design subnets in the cloud which for those in need of network segregation is a key enabler. The ability to route cloud traffic via the VPN past ones favourite local Inspection/Intrusion detection systems is also going to win favour with the Security Architects.
Its unclear from the early (beta) documentation if its possible to firewall routable traffic between subnets in the cloud. You can certainly route traffic back through the corporate firewall. I am sure organisations will want to have the option to route from one application tier to another while firewalling the tiers without having to take an overhead of routing back to base.