Are they offering something else (Cus. service, HA, uptime?) that I can't currently get on AWS?
OpenStack (which HPCloud is based on) has a long way to go, but I think this is a promising (if overdue) start.
If you want to play with OpenStack for free I can recommend https://trystack.org/
But a shared block storage? You mean a read-only one? Otherwise it's a bit difficult to synchronise access in any useful way. I don't know of any FS that could handle a shared RW block storage.
To be honest, I don't know if openstack implementation supports block device sharing at the moment, but there's definitely an open possibility (from the api response fragment): `"attachments": ,` - it's an array for a reason ;) I'd recommend actually checking before you say it's not there...