Edit: I notice that a lot of people think ZFS is a cluster/distributed filesystem. I don't understand where they get this idea.
There is a problem on Amazon EC2, that you can mount EBS (Elastic Block Storage) only to one EC2 instance at a time.
I was told by several people (including some from Sun), that the only way to mount the same EBS instance to multiple EC2 instances is to use OpenSolaris and ZFS. I don't think this solution involves NFS.
Anyway now when ZFS almost ported to Linux, it would be great to check this issue again (requires ZPL).
What might work if you wanted to do this is to mount the EBS volume to an instance and then expose the volume using a network block device to multiple instances. Seems convoluted. Being able to attach EBS to multiple instances may be something Amazon will add if there is enough interest.
ZFS does make it easy to mirror multiple LUNs for redundancy though, so if you have a timeout or permanent disconnect from the device you remain operational. In addition detaching the device and moving it to another machine is trivial with zfs export/import commands.