The 2.6.18 they are using is RHEL-5's version... which is updated (in additional to security updates) about every 6 months. Red Hat back-ports some features and lots of drivers for enterprise hardware... so hardware support generally isn't a problem.
The OpenVZ Project manager did announce that the next stable branch (in the works) will be based on 2.6.32. It appears that Debian, Ubuntu, and RHEL6 will be using that kernel. Not sure when that will be but I would imagine that Proxmox VE will offer a newer stable OpenVZ branch when it becomes available.