Unfortunately OCFS2 requires access to block devices to enabling clustering.

This is something that Openfiler via iscsi was looking like being able to do, unfortunately I think it requires the ability to boot the Xen image with block devices exported into the virtual machine.
http://wiki.rpath.com/wiki/Xen_DomU_Guide

I have been trying to get Openfiler working without success see these articles.
http://blog.dbadojo.com/2007/07/openfiler-fun.html
http://blog.dbadojo.com/2007/07/openfiler-fun-sequel.html
http://blog.dbadojo.com/2007/07/openfiler-on-ec2-much-more-work-and.html

If anyone has any ideas I would love to here them as comments on the blog.

Enormaly are working on a s3 block device which might work.

Given the MySQL 5.1 NDB cluster took all of 15 minutes to setup, and has the ability with enough data nodes to withstand the loss of any one machine it is looking more favourable at the moment.

If you are talking OCFS2 you are looking at Oracle RAC probably on ASM. Given the speed of the interconnect is ultra-important for the shared database cache the current bandwidth would be interesting. Testing and sandbox yes, production no way.
The other issue with OCFS2 is that the box exposing or providing the block device has to be redundant as well otherwise storage becomes a single point of failure.

Have Fun

Paul

My background is as a Database Specialist/DBA, I support Oracle, SQLserver and MySQL
with my mates at Pythian.